To determine if infection with herpes simplex virus (HSV) type 2 is associated with human immunodeficiency virus (HIV) type 1 infection among patients attending sexually transmitted diseases clinics, a case-control study was done on coded sera from 179 HIV-1-infected patients and 367 age-, race-, and gender-matched HIV-1-seronegative patients. Although only 13 (2.3%) of 546 patients had a history of genital herpes treatment, 72% and 56.6%, respectively, had serologic evidence of prior infection with HSV-1 and -2. HSV-1 antibody prevalence was similar among both patient groups; however, HSV-2 antibodies were more common among those infected with HIV-1. Among heterosexual men, 62.7% of those infected with HIV-1 had HSV-2 antibodies compared with 46.7% of those not infected (P less than .01). The HSV-2 seroprevalence among women with or without HIV infection was 78.1% and 57.7%, respectively (P less than .02). A history of intravenous drug use and a reactive serologic test for syphilis were each independently associated with HIV-1 infection in heterosexuals. These data suggest that the two most common causes of genital ulcerative disease in the United States, genital herpes and syphilis, may contribute to increased risk for HIV-1 infection among heterosexuals.