The major cause of chronic post-transfusion hepatitis, the hepatitis C virus (HCV), has been identified. HCV is a single-stranded linear RNA virus with characteristics similar to the flaviviruses. A different agent, the hepatitis E virus, is associated with epidemic (enterically-transmitted) non-A, non-B hepatitis. At present, infection with HCV is recognized by the finding of anti-HCV antibodies, positive in up to 90% of patients with chronic non-A, non-B post-transfusion hepatitis. Antibodies to HCV are detected in 1% of normal volunteer blood donors and in the majority of donors implicated in post-transfusion hepatitis. HCV antibodies are also found in patients with autoimmune liver disease and hepatocellular carcinoma. Moreover, HCV infection may contribute to the pathogenesis of liver disease in alcoholic patients. The role of HCV infection in fulminant non-A, non-B hepatitis and hepatitis-associated aplastic anemia has not been elucidated as yet. Therapy of chronic non-A, non-B hepatitis with recombinant human alpha-interferon has been shown to improve or normalize aminotransferase levels in approximately 50% of patients, most of whom have evidence of HCV infection. However, relapse after cessation of treatment is common. In the future, screening blood for evidence of HCV infection may prevent most cases of non-A, non-B post-transfusion hepatitis.