Concern exists that youth who spend a lot of time participating in organized out-of-school activities (e.g., sports) are at-risk for poor developmental outcomes. This concern--called the over-scheduling hypothesis--has primarily been assessed in terms of adolescent adjustment. This longitudinal study of a nationally representative sample of 1,115 youth (ages 12-18) assessed long-term relations between intensity of participation during adolescence and adjustment at young adulthood (ages 18-24). Time diaries measured intensity as hours per week of participation. Results showed that, controlling for demographic factors and baseline adjustment, intensity was a significant predictor of positive outcomes (e.g., psychological flourishing, civic engagement, and educational attainment) and unrelated to indicators of problematic adjustment (e.g., psychological distress, substance use, and antisocial behavior) at young adulthood. (Contains 2 tables.)