Utilization review (UR) is widely instituted to ensure that medical treatment is clinically necessary and appropriate. UR programs have been criticized for their failure to promote quality and for relying on proprietary review criteria that are rarely subject to external, independent evaluation or validation. In fashioning its UR program for workers’ compensation, the Washington State Department of Labor and Industries sought to address these shortcomings. Working collaboratively with the state medical association, the Department of Labor and Industries developed treatment guidelines and then used these guidelines to formulate review criteria for UR. From 1993 through 1998, 100,005 UR reviews were conducted, half of which used the guideline-based review criteria. We analyzed these reviews to examine the patterns of denied requests. The overall denial rate for the guideline-based reviews was 7.3%. The highest denial rates were for thoracic outlet syndrome surgery (19.1%) and lumbar fusion (17.7%). The use of guideline-based UR protocols may improve the effectiveness of UR as a tool to identify potentially inappropriate care.