Objective: There are presently almost no empirically validated treatments for adolescents with ADHD. However, in childhood, behavioral treatments for ADHD typically include behavioral parent training, classroom interventions, and intensive child-directed interventions. Method: The present investigation examines treatment gains following an 8-week intensive summer day treatment program for adolescents with ADHD (STP-A). Baseline measures of functioning and parent improvement ratings were obtained for 34 STP-A participants. Results: Parent ratings indicated that adolescents who attended the STP-A improved across all target domains. Across domains, 63.0% to 90.9% of adolescents improved during the STP-A. There was no evidence of iatrogenic effects during the STP-A. Correlates of treatment response included adolescent effort, oppositional-defiant behavior, and cognitive/scholastic functioning. Conclusion: Findings are discussed with regard to effective treatment delivery for adolescents with ADHD. (Contains 2 tables.)