Objectives To investigate the impact of early sexual debut on HIV seroprevalence and incidence rates among a cohort of women. Design Prospective study. Setting KwaZulu-Natal, South Africa. Participants A total of 3492 sexually active women who consented to screen a HIV prevention trial during September 2002 to September 2005; a total of 1485 of them were followed for approximately 24 months. Primary and secondary outcome measures HIV seroprevalence among those who were screened for the trial and HIV seroconversion among those who seroconverted during the study. Results Lowest quintiles of age at sexual debut, less than high school education, a higher number of lifetime sexual partners and lack of cohabitation, being diagnosed as having herpes simplex virus 2 and other sexually transmitted infections were all significantly associated with prevalent HIV infection in multivariate analysis. During follow-up, 148 (6.8 per 100 person-years, 95% CI 5.8 to 8.0) women seroconverted. Highest seroconversion rate was observed among women who had reported to have had sex 15 years or younger (12.0 per 100 person-years, 95% CI 8.0 to 18.0). Overall, impact of risk factors considered in this study was associated with considerable potential reductions in HIV prevalence and incidence rates (population attributable risk: 85%, 95% CI 84% to 87% and population attributable risk: 77%, 95% CI 72% to 82%, respectively). Conclusions The association of HIV status with younger age at sexual debut may likely due to an increased number of lifetime partners. This increase could result from longer duration of sexual life. Prevention of HIV infection should include efforts to delay age at first sex in young women. Trial registration number NCT00121459.