Aim: This study investigates the association between two types of social relations during leisure time (to parents and peers) and the frequency of alcohol use among Swedish adolescents, taking possible interaction effects into account. Methods: The data were collected during the 1995-2005 time period by using a questionnaire handed out in the class room. The study includes about 10,000 Swedish adolescents aged 15-16 years. Results: The results show that there are strong associations between the social relations adolescents have during leisure time (both to parents and peers) and the frequency of alcohol use. High levels of peer activity were associated with higher frequencies of alcohol use. Although the effects of relations with parents were modified by peer activity frequencies, high levels of parental monitoring were significantly associated with lower frequencies of alcohol use, regardless of the peer activity frequencies. Conclusions: Parental monitoring is an efficient way to prevent or reduce adolescents' alcohol use, although its importance may vary due to peer activity frequency. (Contains 3 tables.)