We reviewed published and unpublished studies on seroprevalence of HIV antibody in persons attending sexually transmitted disease (STD) clinics in the United States from 1985 through 1987. We identified 23 studies from 16 states; nine studies determined risk factors for HIV. Overall, 899 (4.2%) of the 21,352 clinic attendees were seropositive; the seroprevalence rate was higher for men (5.9%) than for women (1.7%). Clinic seroprevalence ranged from 0.5% to 15.2% (median, 3.5%), reflecting in part the proportion of all attendees who were homosexual or bisexual, intravenous-drug users (IVDUs), or heterosexual partners of bisexual men or IVDUs (median proportion, 21.8% for the nine sites with this information). Most HIV-seropositive persons were at recognized risk (median for the same nine studies, 85.3%). Homosexual/bisexual men had the highest seroprevalence (median, 32.2%), followed by heterosexual IVDUs (median, 3.6%). Heterosexuals who denied intravenous-drug use had a median rate of 0.9%, which strongly correlated with rates in IVDUs in the same clinics (r = 0.88). We conclude many STD clinic attendees are infected with HIV. Because AIDS is an STD and seroprevalence has been associated with other STDs, STD clinics are important sites for HIV surveillance and risk-reduction education.