We studied human immunodeficiency virus (HIV) seroprevalence and risk factors in 3052 clients attending three large public, sexually transmitted disease (STD) clinics in central North Carolina in mid-1988. Anonymous self-administered questionnaires linked to HIV serologies obtained by testing extra blood from syphilis serologies without personal identifiers showed the following characteristics of the respondents: 60% were men, 81% were black, the median age was 24 years, 5% were injecting drug users since 1978, 7% reported a history of syphilis, and 8% of men were homosexual or bisexual. HIV seropositivity was found in 76 subjects (2.5%), including 46% of the homosexual men, 25% of the bisexual men, 1.6% of the heterosexual men, and 0.6% of the women. Elevated HIV seroprevalence rates were found in subjects with a history of or seroreactivity for syphilis (HIV-positive rate of 53% in homosexual or bisexual men, 9% in heterosexual men, 3% in women) and with histories of gonorrhea (HIV-positive rate of 37% in homosexual or bisexual men, 2.6% in heterosexual men, 1% in women), and intercourse (41% in homosexual or bisexual men, 2% in women), prostitute contact (5% in heterosexual men), and sex with casual partners (2% in women). Even a state with a low incidence of acquired immunodeficiency syndrome can include subpopulations with a high HIV seroprevalence, apparently disseminated endemically in association with bacterial STDs.