In Japanese blood donors, positivity for antibodies to hepatitis C virus (HCV) ranges from 0.2% in subjects under 20 to 3.9% in those over 50 years. It is estimated that at least 2.3 million Japanese have contracted HCV infection through contaminated blood. HCV carrier state was confirmed by polymerase chain reaction for HCV-RNA in subjects positive for antibodies to more than one viral protein (70% of cases). Subjects positive for core antibody alone, however, were found to be HCV-RNA negative with normal liver function, and are considered to have only a past history of HCV infection (30% of cases). Acute hepatitis C progresses to chronic infection in about 90% of cases. In comparison with hepatitis B, chronic hepatitis C leads more frequently to cirrhosis and liver cancer, and rarely remits spontaneously. In typical HCV infection, aminotransferase activities fluctuate markedly in the early stages, then become relatively stable for 10 years or more, with chronic persistent hepatitis shown by histological examination. Thereafter, aminotransferase activities may change dramatically, with progression to chronic active hepatitis and rapid development of cirrhosis and hepatocellular carcinoma. On average, it takes about 30 years for chronic hepatitis C to progress from initial infection to cirrhosis and cancer, but the disease progresses much more rapidly in elderly patients.