The Fast Track prevention trial was used to test hypotheses from the Early-Starter Model of the development of chronic conduct problems. We randomly assigned 891 high-risk first-grade boys and girls (51% African American) to receive the long-term Fast Track prevention or not. After 4 years, outcomes were assessed through teacher ratings, parent ratings, peer nominations, and child self-report. Positive effects of assignment to intervention were evident in teacher and parent ratings of conduct problems, peer social preference scores, and association with deviant peers. Assessments of proximal goals of intervention (e.g., hostile attributional bias, problem-solving skill, harsh parental discipline, aggressive and prosocial behavior at home and school) collected after grade 3 were found to partially mediate these effects. The findings are interpreted as consistent with developmental theory.