Hepatitis C virus (HCV) was recognized in 1989 as the main cause of transfusion-related hepatitis non-A, non-B (HNANB) as well as sporadically-acquired HNANB infections. The HCV genome has been sequenced but the virus has not yet been visualized. Anti-HCV antibody tests have been developed, namely enzyme-linked immunosorbent assays (ELISA) for screening and recombinant immunoblot assay (RIBA) for confirmative testing of ELISA positive results. Since May 1991 all Danish blood donations have been tested for anti-HCV antibodies. Approximately 0.04% of Danish blood donors are 4-RIBA positive and have signs of chronic HCV infection based on ALT, HCV-RNA or histological findings. High-risk groups are intravenous drug abusers, haemophiliacs, haemodialysis patients and recipients of multiple blood transfusions. Approximately one third of the cases are sporadic, i.e. without obvious risk factors. Sexual transmission of HCV may occur, but this route is uncommon. More than half of the patients with acute HCV infection progress to chronic hepatitis C. Of these a considerable proportion develops cirrhosis and some eventually hepatocellular carcinoma. At present interferon (IFN) is the only registered therapy against chronic hepatitis C. Approximately half of the patients respond to treatment. The majority, however, relapse after stopping IFN treatment and only 20% maintain their response.