The participants completed primary treatment in the same residential treatment facility between 1989 and 1993. Sources of referrals to the 28-day residential treatment facility included social service agencies, schools, probation officers, families, and others. Data came from clinical records, random urine screens, and interviews with the adolescents and their parents. Results revealed that 61.1 percent of the participants relapsed to pretreatment levels of drug use during the 12 months after treatment. This figure included 79.6 percent of the males and 59.3 percent of the females. A total of 45.9 percent of those with alcohol as the drug of dependence relapsed to the pretreatment levels of use in 12 months. Likewise, 75 percent of marijuana users, 70.6 percent of combined alcohol and marijuana users, and 50.2 percent of other drug users relapsed to pretreatment levels of use in the 12 months. Findings indicate the need to consider gender in assessing treatment effectiveness and designing aftercare programs and the importance of drug-specific findings. Additional research should focus on how interactions among unchangeable characteristics and other factors influence treatment outcomes and on several other issues. Tables, figures, and 21 references (Author abstract modified)