In this piece of research, the authors put forward the hypothesis that closeness of peer group relationships was a critical factor in determining peer influence on substance abuse. This was examined using a panel study of the effects of peer approval and behaviour in relation to cigarette smoking, drinking and other drug use among Irish adolescents. A specific focus of the research was the different roles of 'best friend', 'good friends' and 'same age peers', in the establishment, maintenance and prediction of drug use. 'Good friends', as identified by the respondents, were found to be more influential than 'same age peers' in predicting drug use, but the person identified as 'best friend' was the most influential of all. It was also found that while 'good friends' were influential in terms of example and approval, in the respondent's initiation to drug misuse, the 'best friend' had a critical role in the maintenance of such behaviour.