BACKGROUND: HIV prevention among young people in southern Africa is a public health priority. There is little rigorous evidence of the effectiveness of different intervention approaches. We describe findings of a cluster randomized trial of a community-based, multicomponent HIV, and reproductive health intervention aimed at changing social norms for adolescents in rural Zimbabwe. METHODS: Thirty rural communities were randomized to early or deferred implementation of the intervention in 2003. Impact was assessed in a representative survey of 18-22-year-olds after 4 years. Participants self-completed a questionnaire and gave a dried blood spot sample for HIV and herpes simplex virus-2 (HSV-2) antibody testing. Young women had a urinary pregnancy test. Analyses were by intention-to-treat and were adjusted for clustering. FINDINGS: Four thousand six hundred and eighty-four, 18-22-year-olds participated in the survey (97.1% of eligible candidates, 55.5% women). Just over 40% had been exposed to at least 10 intervention sessions. There were modest improvements in knowledge and attitudes among young men and women in intervention communities, but no impact on self-reported sexual behavior. There was no impact of the intervention on prevalence of HIV or HSV-2 or current pregnancy. Women in intervention communities were less likely to report ever having been pregnant. INTERPRETATION: Despite an impact on knowledge, some attitudes, and reported pregnancy, there was no impact of this intervention on HIV or HSV-2 prevalence, further evidence that behavioral interventions alone are unlikely to be sufficient to reverse the HIV epidemic. The challenge remains to find effective HIV prevention approaches for young people in the face of continued and unacceptably high HIV incidence, particularly among young women.