Chronic hepatitis frequently occurs after liver transplantation. The role of hepatitis C virus infection in patients after liver transplantation is unknown, although antibodies to HCV are detected in some of these cases. The use of polymerase chain reaction techniques for the detection of hepatitis C virus RNA should improve sensitivity and specificity, particularly in these immunosuppressed patients. O u r goal was to further clarify the role of hepatitis C virus infection in chronic hepatitis occurring after liver transplantation. Patients with chronic hepatitis of uncertain origin after transplantation were identified. Serum samples taken at the time of the most recent liver biopsy that showed chronic hepatitis were tested f o r anti-hepatitis C virus using enzyme-linked immunoassay and supplemented by recombinant immunoblot assay (recombinant immunoblot assay I and recombinant immunoblot assay 11). The samples were also tested for the presence of hepatitis C virus RNA using polymerase chain reaction. Of the 25 patients with chronic hepatitis, 15 (60%) had hepatitis C virus RNA present. Only seven (47%) of these 15 patients had anti-hepatitis C virus detected. Hepatitis C virus is a major cause of chronic hepatitis occurring after liver transplantation. The magnitude of hepatitis C virus infection will be underestimated if only currently available assays f o r anti-hepatitis C virus are used.