A total of 41 patients with chronic hepatitis C virus (HCV) defined as abnormal liver injury test results for 6 months or more and HCV RNA positivity in plasma were studied to determine if the liver might not be the only focus of HCV infection in individuals treated with interferon alfa (IFN-α). All patients were examined for the presence of confounding liver disease and tested negatively for such findings. All tested positively for HCV RNA and had an abnormal hepatic histology. All were treated with IFN for 6 months at a dosage of 5 million units daily. After 6 months of therapy, 29 (71%) had normal alanine transaminase (ALT) values, and 25 (61%) tested negatively for HCV RNA. After 6 months of follow-up, without IFN therapy, 17 (41%) still had normal ALT values, and 16 (39%) still tested negatively for HCV RNA in serum. Patients who continued to test negatively for HCV RNA in serum after 6 months of follow-up also tested negatively for HCV RNA in the liver at the end of IFN therapy. Only 2 subjects who tested negatively for HCV RNA in the liver at the end of treatment relapsed after discontinuing IFN therapy. In contrast, patients who tested positively for HCV RNA in the liver after 6 months of therapy relapsed and tested positively for HCV RNA in serum during the 6 months of follow-up. These results suggest that (1) a nonhepatic site of HCV infection may exist; (2) this putative extrahepatic site appears to be less responsive to IFN therapy than in the liver; and (3) this unknown extrahepatic site of infection may be the source of HCV reactivation in cases that relapse during IFN follow-up periods. (HEPATOLOGY 1995; 22:1109–1112.).