Epidemiologic studies link Kaposi's sarcoma with a sexually transmitted agent. Human herpesvirus 8 (HHV-8) is likely to be that agent, but routes of transmission are poorly described. A seroepidemiologic study was conducted to determine whether HHV-8 is transmitted sexually between heterosexuals. Sera from 2718 patients attending a sexually transmitted disease (STD) clinic were tested for antibodies to HHV-8 and herpes simplex virus type 2 (HSV-2). Information on sex partners in the previous 12 months and past STDs were obtained by questionnaire. Relationships between possible risk factors and HHV-8 infection were assessed by logistic regression. Overall, seroprevalence of HHV-8 was 7.3%. Independent risk factors for HHV-8 in the whole group were homo/bisexuality and birth in Africa and, among homo/bisexual men, a history of syphilis and HSV-2 and human immunodeficiency virus seropositivity. Among heterosexuals there was no evidence for sexual transmission; the only independent risk factor for HHV-8 seropositivity was birth in Africa.