Objective: We conducted a large (N = 216) multisite clinical trial of the Challenging Horizons Program (CHP)—a yearlong afterschool program that provides academic and interpersonal skills training for adolescents with attention-deficit/hyperactivity disorder. Intent-to-treat analyses suggest that, as predicted, the CHP resulted in significant reductions in problem behaviors and academic impairment when compared to community care. However, attendance in the CHP ranged from zero to 60 sessions, raising questions about optimal dosing. Method: To evaluate the impact of treatment compliance, complier average causal effect modeling was used to compare participants who attended 80% or more of sessions to an estimate of outcomes for comparable control participants. Results: Treatment compliers exhibited medium to large benefits (ds = 0.56 to 2.00) in organization, disruptive behaviors, homework performance, and grades relative to comparable control estimates, with results persisting 6 months after treatment ended. However, compliance had little impact on social skills. Conclusions: Students most in need of treatment were most likely to comply, resulting in significant benefits in relation to comparable control participants who experienced deteriorating outcomes over time. Difficulties relating to dose-response estimation and the potentially confounding influence of treatment acceptability, accessibility, and client motivation are discussed.