This research examined deviant talk during summer residential treatment using peer nominations and extensive field observations. Participants were 239 youth (M age = 12.62, SD = 2.60; 67% male), nested in 26 treatment groups. Deviant talk was present in this setting, showed individual differences, and increased over time, especially for younger boys. As expected, its relationship to treatment response was moderated by peer behavior. Initial levels of individual deviant talk were related to clinical improvement, but primarily when peer deviant talk was low. Initial levels of peer deviant talk were related to higher than expected end of treatment aggression, especially for youth who were high in deviant talk. Deviant talk effects were observed for staff impressions of change and observations of aggression and adjustment. Initial antisocial behavior affected whether individual or peer levels of deviant talk more heavily influenced treatment response. Implications for clinical assessment and treatment monitoring are discussed.