The association between the development of antisocial behavior, affiliation with deviant friends, and peer rejection was tested with a preventive intervention; 664 boys and girls were randomly assigned to a universal classroom-based intervention targeting disruptive behavior or a control condition. Peer nominations of antisocial behavior, friends' antisocial behavior, and peer rejection were assessed annually for 4 years. A high, a moderate, and a stable low antisocial behavior trajectory were identified. Large reductions in antisocial behavior were found among intervention children who followed the high trajectory. These reductions coincided with affiliations with nondeviant peers and with decreases in peer rejection. The affiliation between deviant and nondeviant peers was initiated by nondeviant children. The results support a causal role of deviant friends and peer rejection in the development of antisocial behavior. The implications for our understanding of the mechanisms leading to reductions in antisocial behavior are discussed.