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UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

FORM 10-Q

For the Quarterly Period Ended September 30, 2008

Commission File No. 0-21886

BARRETT BUSINESS SERVICES, INC.

(Exact name of registrant as specified in its charter)

(360) 828-0700

(Registrant’s telephone number, including area code)

Indicate by check mark whether the registrant (1) has filed all reports required to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during the preceding 12 months (or for such shorter period that the registrant was required to file such reports), and (2) has been subject to such filing requirements for the past 90 days.    Yes  x    No  ¨

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer or a smaller reporting company (as defined in Rule 12b-2 of the Exchange Act).

Large accelerated filer  ¨            Accelerated filer  x            Non-accelerated filer  ¨            Smaller reporting company  ¨

Indicate by check mark whether the registrant is a shell company (as defined in Rule 12b-2 of the Exchange Act).    Yes  ¨    No  x

Number of shares of common stock, $.01 par value, outstanding at October 31, 2008 was 10,685,962 shares.

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BARRETT BUSINESS SERVICES, INC.

INDEX

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Part I - Financial Information

BARRETT BUSINESS SERVICES, INC.

Consolidated Balance Sheets

(Unaudited)

(In thousands, except per share amounts)

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BARRETT BUSINESS SERVICES, INC.

Consolidated Statements of Operations

(Unaudited)

(In thousands, except per share amounts)

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BARRETT BUSINESS SERVICES, INC.

Consolidated Statements of Operations

(Unaudited)

(In thousands, except per share amounts)

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BARRETT BUSINESS SERVICES, INC.

Consolidated Statements of Cash Flows

(Unaudited)

(In thousands)

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BARRETT BUSINESS SERVICES, INC.

Notes to Consolidated Financial Statements (Unaudited)

Note 1 - Basis of Presentation of Interim Period Statements

The accompanying consolidated financial statements are unaudited and have been prepared by Barrett Business Services, Inc. (“Barrett”, “BBSI” or the “Company”), pursuant to the rules and regulations of the Securities and Exchange Commission. Certain information and note disclosures typically included in financial statements prepared in accordance with accounting principles generally accepted in the United States of America have been condensed or omitted pursuant to such rules and regulations. In the opinion of management, the consolidated financial statements include all adjustments, consisting only of normal recurring adjustments, necessary for a fair statement of the results for the interim periods presented. The preparation of financial statements in conformity with generally accepted accounting principles (“GAAP”) requires management to make estimates and assumptions that affect the amounts reported in the financial statements and accompanying notes. Actual results may differ from such estimates and assumptions. The consolidated financial statements should be read in conjunction with the audited financial statements and notes thereto included in the Company’s 2007 Annual Report on Form 10-K at pages F1 - F24. The results of operations for an interim period are not necessarily indicative of the results of operations for a full year.

Effective January 1, 2007, the Company formed a wholly owned captive insurance company, Associated Insurance Company for Excess (“AICE”). AICE is a fully licensed captive insurance company holding a certificate of authority from the Arizona Department of Insurance. The purpose of AICE is twofold: (1) to provide access to more competitive and cost effective insurance markets and (2) to provide additional flexibility in cost effective risk management. The captive principally provides coverage for workers’ compensation claims occurring on or after January 1, 2007. During the second quarter of 2007, AICE began to provide general liability insurance coverage for BBSI on an as requested basis by third parties such as customers, landlords and other vendors.

Financial Statement Reclassification

Certain prior year amounts have been reclassified to present variable rate demand notes (“VRDN”) and municipal bonds as short-term investments instead of cash and cash equivalents. The Company has revised its consolidated balance sheet as of December 31, 2007 and its consolidated statements of cash flows for the nine month period ended September 30, 2007. As a result, the Company’s investments in VRDN and municipal bonds in the amount of $53.0 million at December 31, 2007, which had previously been included in cash and cash equivalents, are presented as investments in the accompanying consolidated balance sheet at December 31, 2007. As a result of this reclassification, the aggregate purchases and proceeds from the sale of these securities for the nine month period ended September 30, 2007 will be presented in the consolidated statements of cash flows from investing activities. These reclassifications had no impact on the Company’s results of operations, changes in shareholders’ equity, or cash flows from operating activities and financing activities.

Our investments in municipal bonds generally consist of bonds from highly rated issuers that are within one to two years of maturity. VRDN are instruments bundled with long-term municipal bonds from highly rated issuers that are traded at par value with an interest rate reset weekly at varying intervals. These instruments include provisions whereby the Company can put the instrument back to the issuer.

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BARRETT BUSINESS SERVICES, INC.

Notes to Consolidated Financial Statements (Unaudited) (Continued)

Note 1 - Basis of Presentation of Interim Period Statements (Continued)

Financial Statement Reclassification (Continued)

The Company had historically classified VRDN and municipal bonds as cash and cash equivalents, which was based on the Company’s ability to liquidate its holdings on short notice and the limited exposure to market volatility of these investments. While our investment strategy continues to emphasize liquidity and close management of market risk, we believe these reclassifications provide greater transparency as to the components of our short-term investments. The effects of these reclassifications are summarized in the table below.

Marketable Securities

As of September 30, 2008, the Company’s marketable securities consisted of tax-exempt municipal securities, Variable Rate Demand Notes (VRDN), closed-end bond funds, equity securities and corporate bonds. The Company classifies municipal securities, VRDN and the closed-end bond funds as available for sale; they are reported at fair value with unrealized gains and losses, net of taxes, shown as a component of other comprehensive income (loss) in stockholders’ equity. In the event a loss is determined to be other-than-temporary, the loss will be recognized in the statement of operations. The equity securities are classified as trading and are reported at fair value with unrealized gains and losses, net of taxes, shown as a component of net income. The corporate bonds are classified as held-to-maturity and are reported at amortized cost.

Allowance for doubtful accounts

The Company had an allowance for doubtful accounts of $399,000 and $100,000 at September 30, 2008 and December 31, 2007, respectively. The Company must make estimates of the collectibility of accounts receivables. Management analyzes historical bad debts, customer concentrations, customer creditworthiness, current economic conditions and changes in customers’ payment trends when evaluating the adequacy of the allowance for

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BARRETT BUSINESS SERVICES, INC.

Notes to Consolidated Financial Statements (Unaudited) (Continued)

Note 1 - Basis of Presentation of Interim Period Statements (Continued)

Allowance for doubtful accounts (Continued)

doubtful accounts. The Company deems an account balance uncollectible only after it has pursued all available assets of the customer and, where applicable, the assets of the personal guarantor.

Workers’ compensation claims

The Company is a self-insured employer with respect to workers’ compensation coverage for all of its employees (including employees subject to Professional Employer Organization (“PEO”) contracts) working in California, Oregon, Maryland and Delaware. In the state of Washington, state law allows only the Company’s staffing services and internal management employees to be covered under the Company’s self-insured workers’ compensation program. To manage our financial exposure, in the event of catastrophic injuries or fatalities, we maintain excess workers’ compensation insurance (through our captive insurance company) with a per occurrence retention of $5.0 million, except in Maryland, where our per occurrence retention is $1.0 million effective January 1, 2007. AICE maintains excess workers’ compensation insurance coverage with AIG between $5.0 million and $15.0 million per occurrence, except in Maryland, where coverage with AIG is between $1.0 million and $25.0 million per occurrence. AIG’s exposure to subprime mortgage securities and recent disruptions in the U.S. financial markets has adversely impacted AIG. However, AIG’s commercial insurance subsidiary had funds in excess of loss reserves at September 30, 2008 and continues to be a fully accepted insurance carrier for all major brokers. As a result, we do not expect these recent developments to have a material impact on our insurance coverage with AIG, but are currently monitoring the situation and evaluating our insurance coverage in light of the situation.

The Company has provided a total of $15.4 million and $12.7 million at September 30, 2008 and December 31, 2007, respectively, as an estimated future liability for unsettled workers’ compensation claims liabilities. Included in the foregoing liabilities are insured claims that will be paid by the Company’s former excess workers’ compensation insurer and for which the Company has reported a receivable from the insurer for the insured claims liability. Insured claims totaled $2.5 million and $2.7 million at September 30, 2008 and December 31, 2007, respectively. The estimated liability for unsettled workers’ compensation claims represents management’s best estimate, which includes an evaluation of information provided by the Company’s internal claims adjusters and our third-party administrators for workers’ compensation claims, coupled with management’s evaluations of historical claims development and conversion factors and other trends. Included in the claims liabilities are case reserve estimates for reported losses, plus additional amounts based on projections for incurred but not reported claims and anticipated increases in case reserve estimates. These estimates are continually reviewed and adjustments to liabilities are reflected in current operating results as they become known.

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BARRETT BUSINESS SERVICES, INC.

Notes to Consolidated Financial Statements (Unaudited) (Continued)

Note 1 - Basis of Presentation of Interim Period Statements (Continued)

Safety incentives liability

Safety incentives represent cash incentives paid to certain PEO client companies for maintaining safe-work practices in order to minimize workplace injuries, thereby meeting certain established loss objectives. The Company has provided $4.9 million and $5.9 million at September 30, 2008 and December 31, 2007, respectively, as an estimate of the liability for unpaid safety incentives. The incentive is based on a percentage of annual payroll and is paid annually to customers who meet predetermined workers’ compensation claims cost objectives. Safety incentive payments are made only after closure of all workers’ compensation claims incurred during the customer’s contract period. The liability is estimated and accrued each month based upon the incentive earned less the then-current amount of the customer’s estimated workers’ compensation claims reserves as established by the Company’s internal and third-party claims administrators, adjusted for expected future development of claims reserves.

Comprehensive income (loss)

Comprehensive income (loss) includes all changes in equity during a period except those that resulted from investments by or distributions to a company’s stockholders. Other comprehensive income (loss) refers to revenues, expenses, gains and losses that under GAAP are included in comprehensive income (loss), but are excluded from net income as these amounts are recorded directly as an adjustment to stockholders’ equity. The Company’s other comprehensive income (loss) is comprised of unrealized holding gains and losses on its publicly traded marketable securities designated as “available-for-sale”, net of realized gains or losses included in net income.

Note 2 - Recent Accounting Pronouncements

In September 2006, the Financial Accounting Standards Board (“FASB”) issued Statement of Financial Accounting Standard (“SFAS”) No. 157, “Fair Value Measurements” (“SFAS 157”). SFAS 157 defines fair value, establishes a framework for measuring fair value in GAAP, and expands disclosures about fair value measurements. SFAS 157 is effective for financial statements issued for fiscal years beginning after November 15, 2007, and interim periods within those fiscal years for financial assets and liabilities. The effective date of the provisions of SFAS 157 for non-financial assets and liabilities, except for items recognized at fair value on a recurring basis, was deferred by the FASB Staff Position SFAS 157-2 and are effective for fiscal years beginning after November 15, 2008. The Company is currently evaluating the impact of the provisions for non-financial assets and liabilities. The adoption of SFAS 157 for financial assets and liabilities has not had a material effect on our consolidated financial statements.

In February 2007, the FASB issued SFAS No. 159, “The Fair Value Option for Financial Assets and Financial Liabilities,” (“SFAS 159”). This statement gives the Company the option to elect to carry certain financial assets and liabilities at fair value with change in fair value recorded in earnings. The adoption of SFAS 159 effective January 1, 2008 has not had a material effect on our consolidated financial statements.

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BARRETT BUSINESS SERVICES, INC.

Notes to Consolidated Financial Statements (Unaudited) (Continued)

Note 2 - Recent Accounting Pronouncements (Continued)

In December 2007, the FASB issued SFAS No. 141(R) “Business Combinations,” (“SFAS 141(R)”). SFAS 141(R) requires the acquiring entity in a business combination to recognize all (and only) the assets acquired and liabilities assumed in the transaction; establishes the acquisition-date fair value as the measurement objective for all assets acquired and liabilities assumed; and requires the acquirer to disclose to investors and other users all the information they need to evaluate and understand the nature and financial effect of the business combination. SFAS 141(R) is effective for fiscal years beginning after December 15, 2008. We are currently evaluating the potential impact of SFAS 141(R) on our consolidated financial statements.

Note 3 - Acquisitions

Effective February 4, 2008, the Company acquired certain assets of First Employment Services, Inc., a privately held staffing company with offices in Tempe and Phoenix, Arizona. The Company paid $3.8 million in cash upon closing and agreed to pay additional consideration of $1.2 million in cash contingent upon the first 12 months of financial performance. The transaction resulted in the recognition of $3.8 million of goodwill, $15,000 of other assets and $15,000 of fixed assets. The Company’s consolidated income statements for the nine months ended September 30, 2008 include First Employment’s results of operations since February 4, 2008.

Effective December 3, 2007, the Company acquired certain assets of Phillips Temps, Inc., a privately held staffing company with an office in downtown Denver, Colorado. The Company paid $1.6 million in cash for the assets of Phillips Temps and the selling shareholders’ noncompete agreements. There was no contingent consideration. The transaction resulted in the recognition of $1.6 million of goodwill, $20,000 of other assets and $8,000 of fixed assets.

Effective July 2, 2007, the Company acquired certain assets of Strategic Staffing, Inc., a privately held staffing services company with five offices in Utah and one office in Colorado Springs, Colorado. The Company paid $12.0 million in cash for the assets of Strategic Staffing and the selling shareholders’ noncompete agreements and agreed to pay additional consideration contingent upon the first 12 months of financial performance. Effective September 4, 2008, subsequent to the financial performance measurement period, we paid an additional $2.0 million in full satisfaction of the contingent consideration. The transaction resulted in the recognition of $13.9 million of goodwill, $60,000 of intangible assets and $117,000 of fixed assets.

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BARRETT BUSINESS SERVICES, INC.

Notes to Consolidated Financial Statements (Unaudited) (Continued)

Note 4 - Basic and Diluted Earnings Per Share

Basic earnings per share are computed based on the weighted average number of common shares outstanding during the period. Diluted earnings per share reflect the potential effects of the exercise of outstanding stock options. Basic and diluted shares outstanding are summarized as follows:

Note 5 – Stock Incentive Plans and Stock-Based Compensation

The Company’s 2003 Stock Incentive Plan (the “2003 Plan”), which provides for stock-based awards to Company employees, non-employee directors and outside consultants or advisors, was approved by shareholders on May 14, 2003. No options have been issued to outside consultants or advisors. The number of shares of common stock reserved for issuance under the 2003 Plan is 600,000. No new grants of stock options may be made under the Company’s 1993 Stock Incentive Plan (the “1993 Plan”). At September 30, 2008, there were option awards covering 190,500 shares outstanding under the 1993 Plan, which, to the extent they are terminated unexercised, will be carried over to the 2003 Plan as shares authorized to be issued under the 2003 Plan. Outstanding options under both plans generally expire ten years after the date of the grant. They were generally exercisable in four equal annual installments beginning one year after the date of grant; however, effective with the close of business on December 30, 2005, the compensation committee of the board of directors accelerated the vesting of all outstanding stock options. Outstanding options which were issued after December 30, 2005 are exercisable in four equal annual installments beginning one year after the date of grant.

The Company applies SFAS No. 123R, “Share-Based Payment” (“SFAS 123R”), to account for compensation expense for options awarded under its stock incentive plans. SFAS 123R requires the grant-date fair value of all share-based payment awards, including employee stock options, to be recognized as employee compensation expense over the requisite service period.

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BARRETT BUSINESS SERVICES, INC.

Notes to Consolidated Financial Statements (Unaudited) (Continued)

Note 5 – Stock Incentive Plans and Stock-Based Compensation (Continued)

The following table summarizes options activity in 2008:

The following table presents information on stock options outstanding for the periods shown:

The fair value of the stock-based awards, as determined under the Black-Scholes model, granted in the nine months ended September 30, 2008, was estimated with the following weighted-average assumptions:

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BARRETT BUSINESS SERVICES, INC.

Notes to Consolidated Financial Statements (Unaudited) (Continued)

Note 6 – Workers’ Compensation

The following table summarizes the aggregate workers’ compensation reserve activity (in thousands):

Note 7 – Fair Value Measurement

The Company has determined that its marketable securities should be presented at their fair value. Fair value is the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. The Company has determined that its closed-end bond funds and equity securities components of its marketable securities fall into the Level 1 category, which values assets at the quoted prices in active markets for the same identical assets. The Company has also determined its municipal bonds and variable rate demand notes components fall into the Level 2 category, which values assets using inputs other than quoted prices that are observable for the asset either directly or indirectly. There were no assets or liabilities where Level 3 valuation techniques were used and there were no assets and liabilities measured at fair value on a non-recurring basis.

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BARRETT BUSINESS SERVICES, INC.

Notes to Consolidated Financial Statements (Unaudited) (Continued)

Note 7 – Fair Value Measurement (Continued)

Marketable securities consist of the following investments (in thousands):

During the third quarter ended September 30, 2008, the Company recorded a non-cash, mark-to-market impairment charge of approximately $3.5 million relating to its investment in four closed-end bond funds. The conclusion to record the impairment charge was based on the continued decline in the market value of the bond funds, as well as reductions in dividends paid by the funds and the decline in the value of the underlying assets held in the funds reported in recent months by the funds’ investment manager. The Company recorded no income tax benefit on this impairment charge given the uncertainty of the Company’s ability to generate future taxable investment gains required to utilize these investment losses. Prior to the Company recording the impairment charge the decline in mark value was carried net of tax in other comprehensive loss.

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BARRETT BUSINESS SERVICES, INC.

Notes to Consolidated Financial Statements (Unaudited) (Continued)

Note 8 – Income Taxes

The effective tax rate differed from the U.S. statutory federal tax rate due to the following:

Note 9 – Credit Agreement

The Company entered into a new credit agreement (the “Credit Agreement”) with its principal bank effective July 1, 2008. The Credit Agreement provides for an unsecured revolving credit facility of up to $7.0 million, which includes a subfeature under the line of credit for standby letters of credit up to $7.0 million as to which there were $5.78 million outstanding at September 30, 2008 in connection with various surety deposit requirements for workers’ compensation purposes. The interest rate on advances against the revolving credit facility, if any, will be, at the Company’s discretion, either (i) equal to the prime rate or (ii) LIBOR plus 1.50%. The Credit Agreement expires July 1, 2009.

Pursuant to the Credit Agreement, the Company is required to maintain compliance with the following covenants: (1) net income after taxes not less than $1.00 (one dollar) on an annual basis, determined as of each fiscal year end, and (2) pre-tax profit of not less than $1.00 (one dollar) on a quarterly basis, determined as of each fiscal end. The Company was in compliance with all covenants at September 30, 2008.

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BARRETT BUSINESS SERVICES, INC.

Overview

Barrett Business Services, Inc. (“Barrett”, the “Company” or “we”), a Maryland corporation, offers a comprehensive range of human resource management services to help small and medium-sized businesses manage the increasing costs and complexities of a broad array of employment-related issues. The Company’s principal services, professional employer organization (“PEO”) services and staffing services, assist its clients in leveraging their investment in human capital. The Company believes that the combination of these two principal services enables it to provide clients with a unique blend of services not offered by the Company’s competition. Barrett’s platform of outsourced human resource management services is built upon expertise in payroll processing, employee benefits and administration, workers’ compensation coverage, effective risk management and workplace safety programs, and human resource administration.

To provide PEO services to a client, the Company enters into a contract to become a co-employer of the client’s existing workforce and Barrett assumes responsibility for some or all of the client’s human resource management responsibilities. PEO services are normally used by organizations to satisfy ongoing human resource management needs and typically involve contracts with a minimum term of one year, renewable annually, which cover all employees at a particular work site. Staffing services include on-demand or short-term staffing assignments, long-term or indefinite-term contract staffing and comprehensive on-site management. The Company’s staffing services also include direct placement services, which involve fee-based search efforts for specific employee candidates at the request of PEO clients, staffing customers or other companies.

The Company’s ability to offer clients a broad mix of services allows Barrett to effectively become the human resource department and a strategic business partner for its clients. The Company believes its approach to human resource management services is designed to positively affect its clients’ business results by:

The Company serves a growing and diverse client base of small and medium-sized businesses in a wide variety of industries through a network of branch offices in California, Oregon, Washington, Idaho, Arizona, Utah, Colorado, Maryland, Delaware and North Carolina. Barrett also has several smaller recruiting offices in its general market areas, which are under the direction of a branch office.

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BARRETT BUSINESS SERVICES, INC.

Results of Operations

The following table sets forth the percentages of total revenues represented by selected items in the Company’s Consolidated Statements of Operations for the three and nine months ended September 30, 2008 and 2007.

We report PEO revenues in accordance with the requirements of Emerging Issues Task Force No. 99-19, “Reporting Revenues Gross as a Principal Versus Net as an Agent” (“EITF No. 99-19”), which requires us to report such revenues on a net basis because we are not the primary obligor for the services provided by our PEO clients to their customers pursuant to our PEO contracts. We present for comparison purposes the gross revenues and cost of revenues information set forth in the table below. Although not in accordance with GAAP, management believes this information is more informative as to the level of our business activity and more illustrative of how we manage our operations, including the preparation of our internal operating forecasts, because it presents our PEO services on a basis comparable to our staffing services.

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BARRETT BUSINESS SERVICES, INC.

Results of Operations (Continued)

The presentation of revenues on the net basis and the relative contributions of staffing and PEO revenues can create volatility in our gross margin percentage. The general impact of fluctuations in our revenue mix is described below.

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BARRETT BUSINESS SERVICES, INC.

Results of Operations (Continued)

A reconciliation of non-GAAP gross PEO revenues to net PEO revenues is as follows: