Recently, factors predicting the response to interferon (IFN) therapy against hepatitis C virus (HCV) have received much attention. To evaluate the usefulness of the quantitation of intrahepatic HCV RNA as a predictive marker of the response to IFN therapy, we compared the amount of intrahepatic HCV RNA with serum levels in 16 patients. Eleven patients who had 10(10) copies/g or more of intrahepatic HCV RNA had increased level of serum alanine aminotransferase (ALT) after IFN therapy, while 4 of 5 patients who had less than 10(10) copies/g of intrahepatic HCV RNA achieved sustained normalization of serum ALT level and were designated as complete responders. Four complete responders possessed significantly less HCV RNA in the liver parenchyma than partial and nonresponders (P = 0.010, Mann-Whitney U-test), but the amount of HCV RNA in the serum was not significantly different between those groups. In conclusion, the results suggest that the quantitation of intrahepatic HCV RNA is a better indicator of the response to IFN therapy than serum HCV RNA.