This study was designed to evaluate the effects of a peer support programme on adolescents' knowledge, attitudes and use of alcohol and tobacco. Year 7 students (average age 12 years) from three schools who offered the programme (n = 169) and from three schools without the programme (n = 157) completed a self-report assessment. Perceptions of their parents' and friends' use of alcohol and tobacco and attitudes towards the participants' use of these substances as well as the participants' own attitudes (preferences and norms) and use of the substances were assessed on three occasions; pre-intervention, post-intervention and at 6 months follow-up. There were no significant effects of the programme on participants' knowledge, attitudes and use of alcohol and tobacco. Over time, participants in both groups reported increased enjoyment of alcohol, increased use of alcohol and tobacco and more lenient attitudes towards these substances. In conclusion, the peer support programme failed to show any positive influence on adolescents' knowledge, attitudes and use of alcohol and tobacco.