Objectives:To determine the risk factors for HIV seroconversion and assess the association between contraceptive use and HIV infection among women attending three large family planning clinics in Dar es Salaam, Tanzania.Design:Prospective cohort study.Methods:Between 1992 and 1995, 2471 HIV-negative women were followed prospectively. Information about sociodemographic characteristics, sexual behavior, contraceptive use and other risk factors was collected at recruitment and updated at follow-up visits. At the end of the study, specimens were collected for HIV testing and laboratory diagnosis of sexually transmitted diseases.Results:The overall HIV incidence was 3.4 per 100 person-years [95% confidence interval (CI), 2.6–4.1]. The risk of HIV seroconversion decreased with increasing age (P= 0.04, test for trend). Women reporting three or more sex partners during the follow-up period had the highest risk of HIV [age-adjusted relative risk (RR), 4.89; 95% CI, 2.6–9.17]. Having an uncircumcised husband was associated with a significantly increased risk of HIV (age-adjusted RR, 3.60; 95% CI, 1.1–11.59). The risk of HIV was also significantly increased among women with gonorrhoea (age-adjusted RR, 3.51; 95% CI, 1.6–7.71) and candidiasis at baseline (age-adjusted RR, 1.98; 95% CI, 1.1–3.33) and among women reporting alcohol consumption during the follow-up period. After controlling for other risk factors, the risk of HIV infection amongst users of oral contraceptive, intrauterine device and injectable contraceptive was not significantly increased. Similarly, there was no significant trend associated with increasing duration of use of any of these contraceptive methods.Conclusion:These findings confirm that a large number of new HIV infections continue to occur in this population. Reassuringly, no significant association was observed between HIV and use of specific contraceptive methods. Interventions to reduce further spread of HIV are still urgently needed.