An anonymous human immunodeficiency virus (HIV) seroprevalence survey was performed on bloods sent for syphilis serologic testing from the general medical or pediatric clinics or emergency room of a municipal hospital in the Bronx, New York City. From July-December 1987, 549 sera from persons aged 15-54 were collected. HIV antibody was detected in 29/549 (5.3%) sera, increased with age from 0% in the group under 20 to 16.7% in those aged 35-39, and was significantly higher in men (27/230, 11.7%) compared to women (2/319, 0.6%) (P less than .05). Among men aged 35-39, 10/29 (34.5%) were HIV infected. The HIV seroprevalence in emergency room sera was 8/61 (13.1%) versus 21/488 (4.3%) from the out-patient clinics (P less than .05). The presence of a reactive syphilis serology was strongly associated with HIV infection independent of gender (Mantel-Haenszel summary odds ratio (OR) 4.1, 95% CI [1.8, 7.7]) but was stronger for women with reactive syphilis serologies (OR 45.5, 95% CI 5.3, 387.6) than for men (OR 2.6, 95% CI 1.2, 5.8). AIDS prevention strategies may reach at-risk sexually active individuals by focusing on hospital-based emergency rooms and out-patients clinics in areas with high HIV seroprevalence.