A seroprevalence study was carried out on 1757 outpatients consecutively seen in a sexually transmitted disease (STD) clinic in order to evaluate the sexual transmission of hepatitis C virus (HCV). A total of 1442 consenting patients were tested for hepatitis C, hepatitis B and human immunodeficiency virus type 1 (HCV. HBV, HIV - 1) antibodies. The relations between anti-HCV, anti-HBc and anti-HIV - 1 were studied. Of 73 anti-HCV positive reactions, 45 (61.6%) were confirmed by the recombinant immunoblot assay (RIBA). The proportion of individuals with anti-HCV was higher in outpatients with a history of sexually transmitted disease than without. It was 2.8% in non drug user heterosexuals and 2.9% in non drug user homosexuals. Intravenous drug users (IDU) had higher anti-HCV prevalence when a history of STD was taken into account (42.3% in subjects with STD versus 36.7% in subjects without STD). Among non drug user heterosexuals an association was found between anti-HCV and anti-HBc. These data suggest that sexual transmission of HCV occurs, although it seems to be less efficient than other parenteral modes of transmission. When a more sensitive and specific marker of HCV infection become available, a more accurate estimate of the frequency and efficiency of the sexual transmission will be possible.