This article explored developmental and intervention evidence relevant of iatrogenic effects in peer-group interventions. Longitudinal research revealed that «deviancy training» within adolescent friendships predicts increase in delinquency, substance use, violence, and adult maladjustment. Moreover, findings from 2 experimentally controlled intervention studies suggested that peer-group intervention increase adolescent problem behavior and negative life outcomes in adulthood, compared with control youth. The data from both experimental studies suggested that high-risk youth are particularly vulnerable to peer aggregations, compared with low-risk youth. We proposed that peer aggregation during early adolescence, under some circumstances, inadvertently reinforces problem behavior. Two developmental processes are discussed that might account for the powerful iatrogenic effects.