['Exposure to antisocial others within treatment group sessions may have negative impact. We extend prior research with adolescents by examining rehabilitation group composition among adult male incarcerated offenders.', 'Data were gathered from institution files of rehabilitation completers (N = 1,832; M age = 33.5; 19% Aboriginal, 68% Caucasian), including general, substance, violent, and sex offenders. Capacities for treatment (including motivation, learning ability, and inhibitory control) were gathered from intake assessments. At the beginning and end of rehabilitation, providers rated program performance. Risk for recidivism and postrelease recidivism were gathered from official files, up to 3 years following release.', 'Group effects accounted for up to 40% of variance in program outcomes. Group features (average group participant risk to reoffend and risk score diversity) significantly interacted with treatment capacities to explain program outcomes. Most models revealed a dampening effect whereby the positive association between capacities and outcome was reduced in groups of higher risk and more risk diverse coparticipants. Group composition typically accounted for 30-38% of variance between groups, but total variance in outcome explained was generally small. Higher average group risk predicted postrelease recidivism among family violence offenders.'] Are rehabilitation group coparticipants ' risk levels associated with offenders ' treatment performance , treatment change , and recidivism?