Hepatitis B and human immunodeficiency virus are transmitted by venereal contact or by the sharing of used needles during drug use. A questionnaire was administered to street youths admitting to prostitution and to a group denying prostitution, as well as to sexually active adolescents who lived with their family and were attending an adolescent medical clinic. The age, gender, race, number of sexual partners, condom use, illicit drug use, anal intercourse and admission to prostitution were recorded for each of the subjects. Serology was obtained for hepatitis B markers and human immunodeficiency virus antibody. Multiple regression analysis was performed to determine the factors which might predict hepatitis B serologic status. A single subject, who admitted to prostitution, was found to be human immunodeficiency virus-seropositive. Seven of 43 (16%) admitted prostitutes compared with 1 of 44 (2%) other street youth and none of 27 controls demonstrated anti-hepatitis B surface antibodies. The higher rate of seropositivity in admitted prostitutes was statistically significant (P less than 0.0097). Two seropositive prostitutes had IgM hepatitis B core antibody suggesting recent infection. Two factors, number of partners (P less than 0.0007) and practice of anal intercourse (P less than 0.05), were identified in the multiple regression analysis as predictive of seropositivity. Thus adolescents who live on the street are at increased risk for becoming infected with hepatitis B virus. The difficulty in ensuring vaccine coverage in this population would support calls for including hepatitis B vaccination as part of childhood immunization.