Hepatitis C virus RNA as detected by reverse transcription and nested polymerase chain reaction was monitored in 16 patients with chronic hepatitis C treated with interferon. Hepatitis C virus RNA became undetectable after 4 to 8 wk of interferon administration in 13 of the 16 patients. During 6 mo of follow-up, 5 of the 13 patients who became negative for hepatitis C virus RNA after interferon administration remained negative, and all five continued to have normal ALT levels. Repeat liver biopsy in these five patients revealed histological improvement. Antibody to hepatitis C virus, which was initially positive in all treated patients, fell to undetectable levels in three of the five patients. In contrast, aminotransferase levels rose again in all eight patients who had become hepatitis C virus RNA negative but had again exhibited hepatitis C virus RNA after completion of therapy. In 16 untreated patients, hepatitis C virus RNA remained detectable. These results indicate that detection of hepatitis C virus RNA may be useful as a marker of viral replication in chronic hepatitis C; they also suggest that interferon should again be administered to patients who become hepatitis C virus RNA negative on treatment but again exhibit this marker of viral replication when treatment is stopped.