AIMS--To investigate whether the amount of hepatitis C viral RNA (HCV-RNA) in liver and serum can predict the effectiveness of interferon treatment in patients with chronic hepatitis C. METHODS--The amount of HCV-RNA in frozen liver tissues and sera of 22 patients with chronic hepatitis C was determined before and after interferon alfa treatment by the competitive reverse transcriptional polymerase chain reaction method. RESULTS--Patients with small amounts of HCV-RNA in serum before treatment showed a significantly more effective response to interferon treatment. After treatment, HCV-RNA disappeared from both the liver and serum of all patients who sustained complete response (n = 11); in contrast, HCV-RNA decreased but persisted in the liver of all those who relapsed after cessation of treatment (n = 11). CONCLUSIONS--The elimination of hepatitis C virus from the liver as well as from the serum seems to be essential for sustained remission. The quantitation of HCV-RNA in liver biopsy specimens obtained after treatment would be a highly accurate predictor of whether relapse is likely to occur.