Hepatitis C virus (HCV) has an etiological role in post-transfusional Non-A Non-B Hepatitis, cirrhosis, and hepatoma. Studies have revealed an high prevalence of anti-HCV antibodies in hemophiliacs, IV drug users, and other groups at risk for parenterally transmitted infections. The authors report findings from their investigation into the sexual transmission of HCV. The prevalences of antibodies to HCV, the hepatitis B core (HBc) antigen, and to Treponema pallidum were assessed among groups of individuals at high and low risk for sexually transmitted diseases (STD). The population at low risk for STDs was comprised of 2494 volunteer blood donors at the Hospital Universitario Clementino Fraga Filho (HUCFF) over the period July-November 1990. The population at high risk for STDs was comprised of 187 adults consecutively enrolled between September 1990 and January 1991 in a cohort study of the natural history of HIV infection. Sera were screened with a first generation HCV ELISA test, with repeat reactive samples further analyzed using a second generation recombinant immunoblot confirmatory test (RIBA-2). Data on the presence of antibodies to HBc, VRDL, and HIV were abstracted from the Blood Bank records. Antibody testing against Treponema pallidum was conducted among HCV-ELISA positive blood donors and their controls using FTA-ABs. 2.08% of blood donors were infected with HCV, 7.96% of the HIV-infected homosexuals, and 8.02% of the whole group with sexually acquired HIV infection. Anti-HBc antibodies were more frequently present in anti-HCV RIBA-2 confirmed positive blood donors than in controls. 33.3% of the HCV-positive blood donors and 11.04% of controls were found to be anti-HBc positive. 17.6% of HCV-positive donors and 4.9% of controls yielded positive FTA-ABs results. 5.9% of samples from blood donors were both anti-HBc and FTA-ABs positive, while none of the controls reacted in both tests. The association between HCV, hepatitis B infection, and syphilis in individuals at low risk for parenterally transmitted diseases suggests that sexual transmission contributes to the maintenance of the endemicity of HCV in the local population.