This study examined the effectiveness of the universal component of the Fast Track prevention model: the PATHS (Promoting Alternative THinking Strategies) curriculum and teacher consultation. This randomized clinical trial involved 198 intervention and 180 comparison classrooms from neighborhoods with greater than average crime in 4 U.S. locations. In the intervention schools, Grade 1 teachers delivered a 57-lesson social competence intervention focused on self-control, emotional awareness, peer relations, and problem solving. Findings indicated significant effects on peer ratings of aggression and hyperactive-disruptive behavior and observer ratings of classroom atmosphere. Quality of implementation predicted variation in assessments of classroom functioning. The results are discussed in terms of both the efficacy of universal, school-based prevention models and the need to examine comprehensive, multiyear programs.