There is a significant excess of serologic evidence of hepatitis type B infection in two high-promiscuity populations: patients with venereal diseases and their unrelated sexual contacts (15% to 18%) and male, but not female, homosexuals (37% to 51%). Spouses of asymptomatic chronic carriers of antigen had a higher prevalence (26% to 28%) than spouses of noncarriers (10% to 11%); however, the prevalence in the former is relatively low when compared with rates seen in other relatives of carriers. Persons who had a higher-than-average probability of exposure to potentially infective partners or whose patterns of sexual behavior made such exposure more likely (large numbers of sexual partners, long duration of homosexuality, involvement in predominantly anal intercourse) were found to have serologic evidence of hepatitis B more frequently than those with other patterns of sexual behavior. This study showed a strong association between serologic evidence of type B hepatitis and patterns of sexual behavior. However, whether or not transmission of hepatitis type B virus occurs through vaginal intercourse could not be ascertained.