Although peer education has enjoyed considerable popularity as a health promotion approach with young people, there is mixed evidence about its effectiveness. Furthermore, accounts of what young people actually do as peer educators are scarce, especially in informal settings. In this paper, we examine the activities of the young people recruited as 'peer supporters' for A Stop Smoking in Schools Trial (ASSIST) which involved 10,730 students at baseline in 59 secondary schools in south-east Wales and the west of England. Influential Year 8 students, nominated by their peers, were trained to intervene informally to reduce smoking levels in their year group. The ASSIST peer nomination procedure was successful in recruiting and retaining peer supporters of both genders with a wide range of abilities. Outcome data at 1-year follow-up indicate that the risk of students who were occasional or experimental smokers at baseline going on to report weekly smoking at 1-year follow-up was 18.2% lower in intervention schools. This promising result was supported by analysis of salivary cotinine. Qualitative data from the process evaluation indicate that the majority of peer supporters adopted a pragmatic approach, concentrating their attentions on friends and peers whom they felt could be persuaded not to take up smoking, rather than those they considered to be already 'addicted' or who were members of smoking cliques. ASSIST demonstrated that a variety of school-based peer educators, who are asked to work informally rather than under the supervision of teaching staff, will engage with the task they have been asked to undertake and can be effective in diffusing health-promotion messages. Given the serious concerns about young people's smoking behaviour, we argue that this approach is worth pursuing and could be adapted for other health promotion messages.