Formerly the diagnosis of acute and chronic non-A, non-B hepatitis was made by the exclusion of other causes. However, in 1989 cloning of an antigenic component of the hepatitis C virus (HCV) was reported. This led to first- and second-generation tests for antibody to HCV (anti-HCV) in serum. HCV has been associated with acute and chronic posttransfusion and sporadic non-A, non-B hepatitis, and with hepatocellular carcinoma. Viral HCV RNA can be estimated with the polymerase chain reaction test, but this technically difficult test is not generally available. The entire viral genome has been sequenced. The envelope region shows considerable variation, and mutant HCV infections are being described already. There are geographic variations in the prevalence of anti-HCV, but usually about 0.5% to 1% of healthy blood donors test positive. Parenteral exposure to blood, especially by transfusion or drug abuse, remains a certain means of acquiring HCV infection. The method by which millions without parenteral risk factors acquire HCV remains uncertain. Vertical transmission and sexual and family spread occur only rarely. Body secretions are free of the virus. The mode of transmission may become clarified when tests for viral HCV as opposed to anti-HCV become generally available. Acute HCV infection usually is mild, and the chronic disease is also indolent. Carriers of hepatitis B virus or alcoholics who also test positive for HCV have more serious disease. Chronic HCV infection must be distinguished from autoimmune chronic active hepatitis. The most important difference is the response to corticosteroid therapy, which is good in autoimmune hepatitis and poor in HCV-related disease. Hepatocellular carcinoma can complicate HCV-related cirrhosis, usually about 20 years after infection with HCV. Recombinant interferon-alpha is used to treat chronic HCV disease, but selection of patients, dose, and duration of therapy are uncertain. In general, 50% of patients respond to the treatment, but 50% of these will have a relapse, with an overall response rate of 25%. Liver transplantation in patients with end-stage HCV disease usually is followed by infection of the graft.