Abstract : OBJECTIVE: To determine if a commonly used violence prevention curriculum, Second Step: A Violence Prevention Curriculum, leads to a reduction in aggressive behavior and an increase in prosocial behavior among elementary school students. DESIGN: Randomized controlled trial. SETTING: Urban and suburban elementary schools in the state of Washington. PARTICIPANTS: Six matched pairs of schools with 790 second-grade and third-grade students. The students were 53% male and 79% white. INTERVENTION: The curriculum uses 30 specific lessons to teach social skills related to anger management, impulse control, and empathy. MAIN OUTCOME MEASURES: Aggressive and prosocial behavior changes were measured 2 weeks and 6 months after participation in the curriculum by parent and teacher reports (Achenbach Child Behavior Checklist and Teacher Report Form, the School Social Behavior Scale, and the Parent-Child Rating Scale) and by observation of a random subsample of 588 students in the classroom and playground/cafeteria settings.