In 1988 investigators defined the agent causing most cases of non-A, non-B hepatitis and named this agent hepatitis C virus (HCV). Assays for antibody to HCV and more specific diagnostic tests (immunoblot and polymerase chain reactions) were subsequently developed. Exposure to HCV seems to result in chronic infection in a majority of cases, with progressive chronic liver disease sometimes leading to hepatocellular carcinoma. This virus is spread mainly by parenteral routes; intravenous drug users are at high risk. Sexual and intrafamilial spread has also been documented but seems to occur to a limited extent. Perinatal transmission, which is prominent in infection with hepatitis B virus, does not play an important role in the spread of HCV. Antiviral therapy with interferon α is usually tried in chronic hepatitis C but permanently normalizes liver function in only one-fourth of patients. The failure to demonstrate protective immunity after single or multiple episodes of infection raises doubts about the existence of neutralizing antibodies and concerns about the potential for the development of a vaccine against HCV.