Pay for performance, the provision of financial incentives for favorable performance, is increasingly under study as an evidence-based practice. This study estimated the improvement in client retention from offering incentives to 11 substance abuse counselors providing outpatient aftercare treatment. During the incentive period, a counselor could earn a bonus of $100, in addition to his regular compensation, for each client who completed at least five aftercare sessions (the "milestone" which we considered the minimum adequate dose of the aftercare curriculum). We evaluated this and a similar, 12-session incentive using a logistic regression in which the retention "milestone" was the dependent variable and the proportion of time in the incentive condition was the independent variable. Among the 123 clients offered this aftercare program, their probability of completing at least 5 sessions was 59% with the incentive compared to 33% beforehand (odds ratio 4.1, P<.01). These findings suggest that counselor incentives are an effective strategy to improve client retention in substance abuse treatment.