Liver biopsies from six patients affected by chronic active hepatitis (CAH) induced by hepatitis C virus (HCV) have been investigated ultrastructurally and their features compared with those of five cases of CAH induced by hepatitis B virus (HBV). Clusters of deeply packed nuclear inclusions (18 to 22 nm in diameter) were found in patients with HCV-CAH. They were irregularly round and ill-delimited. These inclusions were distinct from the regular round and well-delimited nuclear inclusions associated with HBV. HCV-associated nuclear inclusions were similar to a peculiar type of intranuclear particle described in non-A, non-B hepatitis before the HCV disease had been recognized as a distinct entity. These inclusions have never been hitherto reported in infections caused by HBV or other known hepatotropic viruses. Together, these data suggest that the occurrence of these inclusions can be related to HCV activity in hepatocytes. Changes in the microarchitecture of the liver cell and its microenvironment were similar in HCV- and HBV-CAH. They were heterogeneous in different cases and did not display a clear correlation with the severity of the disease.