The effectiveness of adolescent substance abuse treatment has been repeatedly demonstrated, but specific treatment approaches have rarely been sufficiently documented to permit replication. This study evaluated the effectiveness of a manual-guided, outpatient, group-based treatment program for adolescents (N = 194) who were mild-to-moderate substance abusers. In addition to evaluating the group-based treatment model, the study was designed to compare the effectiveness of two approaches to preparing youth to engage in treatment, whereby adolescents received one of two types of treatment induction, either motivational interviewing or counseling overview. Self-reported pretreatment substance use and criminal behaviors were compared with these behaviors 6 and 12 months following treatment entry using a General Linear Mixed Model analytic approach that controlled for the effects of potential confounding variables and examined individual and program factors that might explain treatment response. Participants significantly reduced marijuana use at 6 months, and these reductions were largely sustained at 12 months. No changes in alcohol use or criminal involvement were obtained. Further examination of marijuana use indicated differential treatment response based on participants' emotional abuse history, family satisfaction, school adjustment, and pretreatment substance use frequency. This treatment approach appears promising for marijuana-abusing youth.