Client peer groups in drug abuse treatment programs can be expected to impact on the treatment of drug abusers. Thisex post factostudy explored whether similarity in demographic characteristics and drug use history within client peer groups influences treatment outcome. Subjects were 455 clients in outpatient drug-free treatment. Four aspects of similarity were assessed-age, drug use history, race/ethnicity, and sex. Follow-up data on levels of criminality, nonopiate drug use, and opiate drug use during the first year posttreatment were analyzed using multiple regression analysis. Client similarity was found to influence treatment outcome. While the findings do not warrant substantial changes in the delivery of drug abuse treatment, they suggest that peer relationships and the influence of similarity among clients should be considered in the formulation of treatment plans for individual clients.