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UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

FORM 10-Q

For the Quarterly Period Ended September 30, 2010

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 has submitted electronically and posted on its corporate Website, if any, every Interactive Data File required to be submitted and posted pursuant to Rule 405 of Regulation S-T (§ 232.405 of this chapter) during the preceding 12 months (or such shorter period that the registrant was required to submit and post such files).    Yes  ¨    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).

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 29, 2010 was 10,201,222 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)

The accompanying notes are an integral part of these financial statements

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BARRETT BUSINESS SERVICES, INC.

Consolidated Statements of Operations

(Unaudited)

(In thousands, except per share amounts)

The accompanying notes are an integral part of these financial statements

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BARRETT BUSINESS SERVICES, INC.

Consolidated Statements of Operations

(Unaudited)

(In thousands, except per share amounts)

The accompanying notes are an integral part of these financial statements

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BARRETT BUSINESS SERVICES, INC.

Consolidated Statements of Cash Flows

(Unaudited)

(In thousands)

The accompanying notes are an integral part of these financial statements

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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 2009 Annual Report on Form 10-K at pages F1 - F28. The results of operations for an interim period are not necessarily indicative of the results of operations for a full year. Certain prior year amounts have been reclassified to conform with the current year presentation. Such reclassifications had no impact on net income or stockholders’ equity.

Effective January 5, 2010, the Company formed a wholly owned insurance company, Ecole Insurance Company (“Ecole”). Ecole is a fully licensed insurance company holding a certificate of authority from the Arizona Department of Insurance. Ecole provides workers’ compensation coverage to the Company’s employees working in Arizona for claims occurring on or after March 1, 2010. The cost to capitalize the insurance company was approximately $6.1 million and is included in restricted marketable securities and workers’ compensation deposits in the consolidated balance sheet.

Revenue recognition

We recognize revenue as services are rendered by our workforce. Staffing services are engaged by customers to meet short-term and long-term personnel needs. 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, which cover all employees at a particular work site. Our PEO contracts are renewable on an annual basis and typically require 30 days’ written notice to cancel or terminate the contract by either party. Our PEO contracts provide for immediate termination upon any default of the client regardless of when notice is given. We report PEO revenues in accordance with the current accounting guidance for revenue recognition, 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. Consequently, our PEO service fee revenues represent the gross margin generated from our PEO services after deducting the amounts invoiced to PEO customers for direct payroll expenses such as salaries, wages, health insurance and employee out-of-pocket expenses incurred incidental to employment and safety incentives. These amounts are also excluded from cost of revenues. PEO service fees also include amounts invoiced to our clients for employer payroll-related taxes and workers’ compensation coverage.

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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)

Marketable securities

As of September 30, 2010, the Company’s marketable securities consisted of tax-exempt municipal securities, variable rate demand notes (“VRDN”), equity securities and corporate bonds. The Company classifies municipal securities, VRDN, and certain of its corporate bonds as available for sale; they are reported at fair value with unrealized gains and losses, net of taxes, shown as a component of accumulated 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 shown as a component of net income. Certain of the Company’s 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 $414,000 and $516,000 at September 30, 2010 and December 31, 2009, respectively. The Company must make estimates of the collectibility of accounts receivable. 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 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, Delaware and Colorado. 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. Additionally, effective January 5, 2010, we incorporated a wholly-owned fully licensed insurance company in Arizona to provide workers’ compensation coverage to our employees in Arizona.

To manage our financial exposure, in the event of catastrophic injuries or fatalities, we maintain excess workers’ compensation insurance through our wholly owned captive insurance company, Associated Insurance Company for Excess (“AICE”), with a per occurrence retention of $5.0 million, except in Maryland and Colorado, where our per occurrence retention is $1.0 million and $500,000, respectively. 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, and in Colorado, where the coverage with AIG is between $500,000 and statutory limits per occurrence. We continue to evaluate the financial capacity of our insurers to assess the recoverability of the related insurer receivables.

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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)

Workers’ compensation claims (Continued)

The Company has provided a total of $30.0 million and $27.8 million at September 30, 2010 and December 31, 2009, 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 $2.7 million at September 30, 2010 and December 31, 2009. The estimated liability for unsettled workers’ compensation claims represents management’s best estimate based upon an actuarial valuation provided by a third party actuary. 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.

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 agreed-upon loss objectives. The Company has provided $4.8 million at September 30, 2010 and $4.4 million at December 31, 2009 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. Comprehensive income (loss) totaled $3.7 million and $3.0 million for the three months ended September 30, 2010 and 2009 and $4.2 million and $(6.8) million for the nine months ended September 30, 2010 and 2009, respectively. 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.

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BARRETT BUSINESS SERVICES, INC.

Notes to Consolidated Financial Statements (Unaudited) (Continued)

Note 2 - Recent Accounting Pronouncements

Variable Interest Entities - In June 2009, the FASB amended previously issued authoritative guidance on the consolidation of variable interest entities. The amended guidance is effective for reporting periods beginning after November 15, 2009. The new guidance requires revised evaluations of whether entities represent variable interest entities, ongoing assessments of the primary beneficiary who exercises control over such entities and is required to consolidate the variable interest entity and additional disclosure when the reporting entity holds a variable interest in a variable interest entity but is not the primary beneficiary. Determinations of whether the Company holds a variable interest in a variable interest entity and the determination of who was the primary beneficiary that were made previously are not allowed to be carried forward. The adoption of the new authoritative guidance effective January 1, 2010 has not had a material effect on our consolidated financial statements.

Note 3 - 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:

As a result of the net loss reported for the nine months ended September 30, 2009, 155,521 potential common shares have been excluded from the calculation of diluted loss per share because their effect would be anti-dilutive.

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BARRETT BUSINESS SERVICES, INC.

Notes to Consolidated Financial Statements (Unaudited) (Continued)

Note 4 - Stock Incentive Plans and Stock-Based Compensation

The Company’s 2009 Stock Incentive Plan (the “2009 Plan”), which provides for stock-based awards to Company employees, non-employee directors and outside consultants or advisors, was approved by shareholders on May 13, 2009. No options have been issued to outside consultants or advisors. The number of shares of common stock reserved for issuance under the 2009 Plan is 1,000,000, of which the aggregate number of shares for which incentive stock options may be granted under the Plan is 900,000. No new grants of stock options may be made under the Company’s 2003 Stock Incentive Plan (the “2003 Plan”). At September 30, 2010, there were option awards covering 400,819 shares outstanding under the 2009 Plan, together with the 2003 Plan and its predecessor, the 1993 Stock Incentive Plan. Outstanding options under all three plans generally expire ten years after the date of grant. Options are generally exercisable in four equal annual installments beginning one year after the date of grant.

In accordance with the current accounting guidance for share-based payments, the Company recognizes compensation expense for options awarded under its stock incentive plans. Current accounting guidance 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.

The following table summarizes options activity in 2010:

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BARRETT BUSINESS SERVICES, INC.

Notes to Consolidated Financial Statements (Unaudited) (Continued)

Note 4 - Stock Incentive Plans and Stock-Based Compensation (Continued)

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

The aggregate intrinsic value of stock options represents the difference between the Company’s closing stock price at the end of the period and the relevant exercise price multiplied by the number of options outstanding at the end of the period at each such price.

The fair value of the stock-based awards, as determined under the Black-Scholes model, granted in the nine months ended September 30, 2010 and 2009 were $486,000 and $349,000 or $5.59 and $4.39 per share, respectively, estimated with the following weighted-average assumptions:

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BARRETT BUSINESS SERVICES, INC.

Notes to Consolidated Financial Statements (Unaudited) (Continued)

Note 4 - Stock Incentive Plans and Stock-Based Compensation (Continued)

The following table summarizes stock-based compensation expense related to stock option awards for the three months and nine months ended September 30, 2010 and 2009 (in thousands):

As of September 30, 2010 unamortized compensation expense related to stock options was $642,000.

Note 5 - Workers’ Compensation

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

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BARRETT BUSINESS SERVICES, INC.

Notes to Consolidated Financial Statements (Unaudited) (Continued)

Note 5 - Workers’ Compensation (Continued)

During the second quarter of 2009, the Company engaged a new actuary to review its workers’ compensation liabilities. While the Company historically obtained an actuarial study, management determined the study was not the best estimate of the workers’ compensation liability. Based upon discussions with the new actuary and a thorough review of the Company’s reserving process and consideration of recent developments, management determined the actuarial estimate as of June 30, 2009 was the best estimate of the ultimate cost to settle open claims. Our primary considerations included the significant erosion in the economy, the increasing complexity and uncertainty surrounding healthcare costs, unexpected development in open claims and growth in our business. The change in estimate resulted in the Company increasing its workers’ compensation claims liabilities by approximately $11.8 million at June 30, 2009.

Note 6 - 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.

Factors used in determining the fair value of our financial assets and liabilities are summarized into three broad categories:

The inputs or methodology used for valuing securities are not necessarily an indication of the risk associated with investing in those securities.

The Company has determined that its closed-end bond funds and equity securities components of its marketable securities fall into the Level 1 category. The Company has also determined that its municipal bonds, VRDN and corporate bonds components fall into the Level 2 category. 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 6 - Fair Value Measurement (Continued)

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

During the third quarter ended September 30, 2010, the Company realized a $181,000 gain on the sale of certain closed-end bond funds. During the nine months ended September 30, 2010, the Company realized a total of $484,000 in gains on the sales of certain closed-end bond funds.

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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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Results of Operations

The following table sets forth 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, 2010 and 2009.

We report PEO revenues in accordance with the current accounting guidance for revenue recognition, 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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Results of Operations (Continued)

The presentation of revenues on a 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.