This study hypothesized that closeness of peer group relationships would be a critical factor in determining peer influence on substance use. The hypothesis was examined in a panel study of the effects of peer approval and behaviour on cigarette smoking, drinking and other drug use among Irish adolescents. In support of the closeness hypothesis, the influences relating to peers identified by respondents as ‘friends' were better predictors of drug use than were the corresponding factors relating to same‐aged peers, while the person identified as the ‘best friend’ seemed uniquely influential. Furthermore, there were important differences associated with maintenance of drug use as opposed to initiation, both as regards the type of friends and the mediating factors that were important. In particular, it emerged that several good friends were influential (by means of example and approval) in initiation, while the best friend had a critical role in the maintenance of drug use. 1991 The British Psychological Society