Many interventions for children's behavior problems successfully utilize a group format for social skills training, providing opportunities for practice and performance feedback from peers. Recent studies however, suggest that grouping aggressive children together may reduce intervention effectiveness or even increase risk. The present study examined the relative impact of children's own behavior and their experiences with peers in the first-grade "friendship groups" of Fast Track, a multi-component preventive intervention program. Two-hundred sixty-six children (56% minority, 29% female) participated in 55 friendship groups. Children's own positive and negative behavior in friendship groups was related to relative improvements in social cognitive skills, prosocial behavior, and aggression, assessed through child interviews, teacher ratings, and peer sociometric nominations. Results from hierarchical linear models also revealed that the amount of peer escalation children received for their disruptive behavior during sessions impeded some intervention gains, whereas mere exposure to other children's positive or negative behavior was rarely related to outcomes.KEY WORDS: aggressive behavior; at-risk children; social skills training; group processes; peer relations.