To study the distribution of hepatitis C virus (HCV) in liver tissue and its pathogenesis, nonisotopic in situ hybridization with a digoxin labelled cDNA probe to 5'-NC region of HCV and immunohistochemical study with monoclonal anti-body of anti HCV C33c were performed on the liver tissue from 24 HCV infection patients with chronic liver disease. The results were as follows: In the liver tissue of 17 patients with HCV infection (both ELISA anti-HCV and nested PCR HCV RNA positive in serum), the positivity of HCV RNA and C33c were 82.3% (14/17) and 88.2% (15/17) respectively. In 7 patients with anti-HCV positive only HCV RNA was not detected in the liver tissue, but 4 out of these 7 cases were C33c positive. The specific signals of HCV RNA and C33c were mainly localized in the cytoplasm of hepatocytes. The distribution of the HCV infected hepatocytes was spotty, scattered or diffuse. There was no relationship between the distribution and the level of alanine transaminase or pathologic changes in liver. There were many lymphocytes and monoclear cells infitrating the periphery of HCV-infected liver cells. The results suggested that replication of HCV in the cytoplasm of hepatocytes elicits the host immune response in cells infected with HCV and this is important in the pathogenesis of HCV infection.