The relative role of hepatitis C virus and hepatitis B virus in hepatitis B surface antigen-negative hepatocellular carcinoma was evaluated by polymerase chain reaction in 31 patients from Taiwan. Twenty-one were positive for antibody to hepatitis C virus (group 1) and 10 were negative (group 2). Of the group 1 patients, hepatitis C viral RNA was detected in the serum by polymerase chain reaction in 16 and in the liver tissue in 17, whereas hepatitis B viral DNA was found in the liver tissue in only 4, and none were found in the serum. In group 2 patients, hepatitis C viral RNA was detected in the serum of 1 and in the liver tissue of another. In contrast, hepatitis B viral DNA was found in the serum of 4 patients and in the liver tissues of 5. It was concluded that hepatitis C virus plays an important role in hepatocarcinogenesis in hepatitis B surface antigen-negative patients in Taiwan, especially in those who had antibody to hepatitis C virus; in those without antibody to hepatitis C virus, hepatitis B virus might still be associated with the development of hepatocellular carcinoma in a significant proportion of such patients.