HMOs are under increasing pressure to expand benefits and services for treatment of adolescents who abuse alcohol and drugs. Little information exists, however, on these programs. This article describes a comprehensive adolescent chemical dependency treatment program within an HMO and presents data on use, costs, and results. Characteristics of clients were similar to adolescents seen in community treatment programs. Less than 1% of the HMO adolescent population sought treatment, and the mean number of outpatient treatment visits was 9.7. The median was between two and three visits, and the mode was one visit. Thirty-four percent of the adolescents required residential treatment, and 65% of the adolescents completed the recommended 28-day stay. The additional premium cost per member per month for adolescent chemical dependency treatment was approximately $0.28. A telephone follow-up survey of a random sample of treated adolescents found that most adolescents had reduced their use of alcohol and drugs and had made improvements in other areas of their lives. Few adolescents, however, met the program's goal of total abstinence.