OBJECTIVE To assess the feasibility of conducting a large randomised controlled trial (RCT) of peer led intervention in schools to reduce the risk of HIV/STD and promote sexual health. METHODS Four secondary schools in Greater London were randomly assigned to receive peer led intervention (two experimental schools) or to act as control schools. In the experimental schools, trained volunteers aged 16-17 years (year 12) delivered the peer led intervention to 13-14 year old pupils (year 9). In the control schools, year 9 pupils received the usual teacher led sex education. Questionnaire data collected from year 9 pupils at baseline included views on the quality of sex education/intervention received, and knowledge and attitudes about HIV/AIDS and other sexual matters. Focus groups were also conducted with peer educators and year 9 pupils. Data on the process of delivering sex education/intervention and on attitudes to the RCT were collected for each of the schools. Analysis focused on the acceptability of a randomised trial to schools, parents, and pupils. RESULTS Nearly 500 parents were informed about the research and invited to examine the study questionnaire; only nine raised questions and only one pupil was withdrawn from the study. Questionnaire completion rates were around 90% in all schools. At baseline, the majority of year 9 pupils wanted more information about a wide range of sexual matters. Focus group work indicated considerable enthusiasm for peer led education, among peer educators and year 9 pupils. Class discipline was the most frequently noted problem with the delivery of the peer led intervention. CONCLUSIONS Evaluation of a peer led behavioural intervention through an RCT can be acceptable to schools, pupils, and parents and is feasible in practice. In general, pupils who received the peer led intervention responded more positively than those in control schools. A large RCT of the long term (5-7 year) effects of this novel intervention on sexual health outcomes is now under way.