A cohort of 418 lower socioeconomic strata prostitutes were enrolled in a study of the epidemiology of sexually transmitted diseases (STDs) between January and April 1985. Sixty-two per cent of the women were seropositive for HIV infection at enrollment. Significant associations were found between HIV seropositivity and Tanzanian origin (OR= 2.12, Cl 95%= 1.18–3.81, P< 0.03), younger age, a shorter duration of prostitution, reduced fecundity, use of oral contraceptives (OR= 1.8, Cl 95%= 1.1–2.9, P< 0.05) and genital ulcer disease (OR= 3.32, P< 0.00001). No associations were noted with other STD. Stepwise logistic regression analysis confirmed independent associations between HIV infection and Tanzanian origin (OR= 2.27, Cl 95%= 1.25–4.14, P< 0.007), reduced fecundity (OR= 0.83, Cl 95%= 0.74–0.94, P< 0.003), oral contraceptive use (OR= 2.02, Cl 95%= 1.22–3.35, P< 0.006) and duration of prostitution (OR= 0.39, Cl 95%= 0.23–0.65, P< 0.004). Oral contraceptives may increase susceptibility to HIV or may be a marker for other factors which increase risk of acquisition. Further studies are necessary to confirm this association.