Abstract: In this study, the effects on young adolescent sexual risk behaviour of teacher-led school HIV prevention programmes were examined in two sites in South Africa (Cape Town and Mankweng) and one site in Tanzania (Dar es Salaam). In Cape Town, Dar es Salaam and Mankweng, 26, 24 and 30 schools, respectively, were randomly allocated to intervention or comparison groups. Primary outcomes were delayed sexual debut and condom use among adolescents aged 12–14 years (grade 8 in South Africa and grades 5 and 6 in Tanzania). In total, 5352, 4197 and 2590 students participated at baseline in 2004 in Cape Town, Dar es Salaam and Mankweng, respectively, and 73% (n=3926), 88% (n=3693) and 83% (n=2142) were retained 12–15 months later. At baseline, 13% (n=224), 5% (n=100) and 17% (n=164) had had their sexual debut, and 44% (n=122), 20% (n=17) and 37% (n=57) of these used a condom at last sex, respectively. In Dar es Salaam, students in the intervention were less likely to have their sexual debut during the study (OR 0.65, 95% CI 0.48–0.87). In Cape Town and Mankweng, the intervention had no impact. The current interventions were effective at delaying sexual debut in Dar es Salaam but not in South Africa, where they need to be supplemented with programmes to change the environment in which adolescents make decisions about sexual behaviour.