Hepatitis C virus (HCV) has been associated with acute and chronic posttransfusion and with sporadic non-A non-B (NANB) hepatitis, cirrhosis, and hepatocellular carcinoma (HCC). Cloning of the sequence encoding an antigenic component of HCV in 1989 led to the development of tests to detect antibody to HCV in serum. Viral HCV RNA can be detected and estimated with polymerase chain reaction (PCR) and branched-chain DNA (bDNA) signal amplification tests. The entire viral genome has been sequenced. The HCV envelope region varies considerably, and infections with mutant HCV have been described. Approximately 0.5-1.5% of healthy blood donors test positive, and HCV infection can be acquired by blood transfusion or i.v. drug abuse. Vertical and sexual transmission of the virus is rare, and the transmission mode remains obscure in a large group of patients. Acute hepatitis C is mild and often asymptomatic. Chronic hepatitis C has an indolent course but may progress to cirrhosis and HCC. Recombinant alpha interferon (IF) is used to treat chronic HCV disease, but no consensus has been reached on patient selection, dose, and duration of treatment. Approximately 50% of treated patients respond, but 50-80% of responders relapse over time. Liver transplantation in patients with end-stage, HCV-related liver disease is often followed by allograft infection. Short-term survival with reinfection is good, but the long-term consequences remain to be defined.