The MEMA kwa Vijana (Tanzania) and Regai Dzive Shiri (Zimbabwe) adolescent sexual and reproductive health intervention trials focused on developing skills and changing attitudes and self-efficacy to change behaviours. The trials show a clear gap between young people's knowledge, and their reported attitudes and behaviour. These results suggest that even carefully designed interventions delivered through schools or community peer-educators, with complementary interventions in the health services, are unlikely, on their own, to have a substantial effect on the HIV epidemic in the short or medium term.