BACKGROUND The incidence of genital herpes is increasing worldwide. Herpes simplex virus type 2 (HSV-2) is the most common cause of genital ulcerations among the sexually transmitted diseases (STDS). In order to investigate the sexual transmission of genital herpes infection in Brazil, the prevalence of specific antibodies to HSV-2, in populations of high and low risk for STDS was evaluated. METHODS The population at low risk for STDS was composed of 155 voluntary blood donors at the University Hospitals between February and August, 1994. The population at high risk for STDS was composed of 85 HIV-seropositive persons, consisting of homosexual and promiscuous heterosexual men. A group of 20 highly active prostitutes was also studied between May and July, 1994. All blood samples were screened using an enzyme-linked immunosorbent assay (ELISA) that detects type-specific antibodies to HSV-2 glycoprotein G. Before donating blood, all persons in the study group were given a questionnaire concerning risk factors for acquisition of STDS and suitable to show the socioepidemiologic characteristics of the subjects. RESULTS The prevalence of ELISA showing HSV-2 infections among HIV-seroposite persons, was 73% (P < 0.01). In the whole group at high risk for STDS, the prevalence was 72% (P < 0.05). Infection with HSV-2, as determined by seroprevalence, was significantly and independently associated with years of sexual activity, history of previous STDS, number of sexual partners in the previous month, number of pregnancies, and previous induced abortions, as well as the percentage of sexual acts involving receptive anal intercourse. CONCLUSIONS The current strategy for diagnosing genital HSV-2 infection misses many cases. Newly developed type-specific serologic methods can identify cases of unrecognized or subclinical infection. With the high seroprevalence rates detected, routine screening for HSV-2-specific antibodies should be considered in populations at high risk for STDS.