Objectives-To develop and teach a school sex education programme that will lead to a decrease in sexual activity.Design-A matched internal and external control experiment, comparing control populations which received their own sex education programmes with populations which received a novel sex education intervention that included medical and peer led teaching.Setting-Comprehensive secondary schools; control and intervention populations within Devon, and distant controls from rural, semiurban, and urban areas of England excluding major conurbations.Subjects-Schoolchildren were taught from age 12 to 16; three successive cohorts of students were evaluated in school year 11 (mean age 16.0).Main outcome measures-Questionnaire conducted under "examination conditions" and invigilated by the research team and other trained medical staff. Results-In the intervention population, progressive increase in knowledge related to contraception, sexually transmitted diseases, and prevalence of sexual activity (X2 (trend) P<0-001 for all three series); relative increase between intervention and control populations in knowledge, relative decrease in attitudes suggesting that sexual intercourse is of itself beneficial to teenagers and their relationships, relative decrease in sexual activity, and relative increase in approval of their "sex education" (relative risk > 1-00 with 95% confidence limits not including 1 00 for all series and for comparisons with both control populations); odds ratio (control v programme) for sexual activity of 1*45, controlling for sociodemographic variables.Conclusion-School sex education that includes specific targeted methods with the direct use of medical staff and peers can produce behavioural changes that lead to health benefit.