Two manually guided brief interventions were evaluated with a randomized, controlled trial. Adolescents (aged 13-17 years) suspected of abusing alcohol and other drugs and their parent were randomly assigned to receive either a 2-session adolescent only (BI-A), 2-session adolescent and additional parent session (BI-AP), or assessment only control condition (CON). Adolescents were identified in a school setting, and the intervention was delivered by trained counselors. Outcome analyses (N = 284; 90% of those enrolled) of relative change (from intake to 12 months) and absolute status (at 12 months) revealed a general pattern of reductions in drug use behaviors, particularly with the cannabis outcome measures, in both active conditions (BI-A and BI-AP). Students in the control condition showed worse outcome compared with the BI-A and BI-AP groups. Among the 4 mediating variables measured at 6 months, use of additional services, motivation to change, and parenting practices had significant influences on 12-month outcome; problem-solving skills approached significance as a mediator. The potential value of a brief intervention for drug abusing adolescents is discussed.