The present study was designed to assess the influence of deviant peer affiliations on crime and substance use in adolescence/young adulthood. Data were used from a 21-year longitudinal study of health, development, and adjustment of a birth cohort of 1,265 New Zealand children. Annual assessments of deviant peer affiliations were obtained for the period from age 14-21 years, together with measures of psychosocial outcomes including, violent crime, property crime, alcohol abuse, cannabis abuse, and nicotine dependence. Affiliating with deviant peers was found to be significantly associated with each of these outcomes (p < .0001). Statistical control for confounding by both fixed and time dynamic factors reduced the strength of association between deviant peer affiliations and outcome measures. Nevertheless, deviant peer affiliations remained significantly associated (p < .0001) with all outcomes. For violent/property crime, cannabis and alcohol abuse there was significant evidence of age-related variation in the strength of association with deviant peer affiliations, with deviant peer affiliations having greater influence on younger participants (14-15 years) than older participants (20-21 years). These results suggest that deviant peer affiliations are associated with increased rates of a range of adjustment problems in adolescence/young adulthood with deviant peer affiliations being most influential at younger ages.