An evaluation of the effectiveness of a school-based social competence curriculum PATHS (Promoting Alternative Thinking Strategies) on teacher-rated aggressive behavior, ADHD, and prosocial behavior in children. The one-year prevention program was administered to children in 28 of 56 Swiss elementary schools (N = 1,675). Outcomes were assessed at pretest and posttest with a follow-up 2 years later. Moderator interactions involving baseline child characteristics and economic factors were tested. There were significant treatment effects for ADHD/impulsivity and aggression at the follow-up. Baseline development variables predicted higher prosocial behavior as well as lower aggressive behavior and ADHD at the follow-up. Economic risk factors predicted poor behavior outcomes at the follow-up. Development variablesmoderated the impact of PATHS on ADHD and aggression at the follow-up. However, for most outcomes, no main effects or moderation of treatment effects were found.