Women attending family planning clinics in western Pennsylvania were enrolled into a prospective epidemiologic study of herpes simplex virus (HSV) infection. Detection of antibodies to HSV-1 and HSV-2 was based on an immunodot assay using type-specific glycoproteins gG-1 and gG-2. Serologic and historical data at enrollment were analyzed for 4527 subjects; the seroprevalence of HSV-2 was 21.6%. By multivariate analysis, HSV-2 infection as determined by seroprevalence was significantly and independently associated with age, years of sexual activity, race, one or more episodes of other genital infections, lower annual family income, and multiple sexual partners. The presence of antibody to HSV-1 was associated with a lower frequency of antibody to HSV-2, suggesting that developed immunity to HSV-1 protected against HSV-2 infection. Of 926 participants who had antibody to HSV-2, only 117 (12.6%) reported a history of genital herpes. Hence the vast majority of HSV-2 infections were inapparent. Although seroprevalence of HSV-2 was higher among black (35.4%) than white (18.5%) women, black women appeared to have significantly less symptomatic genital herpes than white women (7.7% vs. 14.7%, P less than .01).