This study evaluates the effects of the Coping Power Program with at-risk preadolescent boys at the time of transition from elementary school to middle school. Aggressive boys were randomly assigned to receive only the Coping Power child component, the full Coping Power Program with parent and child components, or a control condition. Results indicated that the Coping Power intervention produced lower rates of covert delinquent behavior and of parent-rated substance use at the 1-year follow-up than did the control cell, and these intervention effects were most apparent for the full Coping Power Program with parent and child components. Boys also displayed teacher-rated behavioral improvements in school during the follow-up year, and these effects appeared to be primarily influenced by the Coping Power child component.