The first estimates of the seroincidence of human immunodeficiency virus type 1 (HIV-1) and of the risk factors for seroconversion in a cohort of high-risk patients attending sexually transmitted disease (STD) clinics in India are reported. Between 1993 and 1995, 851 HIV-1-seronegative persons were evaluated prospectively every 3 months for HIV infection and biologic and behavioral characteristics. The overall incidence of HIV-1 was 10.2/100 person-years (95% confidence interval, 7.9-13.1). The incidence among commercial sex workers (CSWs) was 26.1/100 person-years, compared with 8.4 among non-CSWs. Recurrent genital ulcer disease and urethritis or cervicitis during the follow-up period were independently associated with a 7- (P < .001) and 3-fold (P = .06) increased risk of HIV-1 seroconversion, respectively. Because of the association of recurrent ulcerative and nonulcerative STDs with HIV-1 seroconversion in this setting, behavioral and biologic measures directed at the prevention and control of STDs would be expected to greatly reduce the transmission of HIV-1 infection in similar high-risk groups.