Normalization of serum aminotransferase levels is achieved in approximately 50% of chronic hepatitis C patients treated with interferon (IFN); however, in about one-half of these patients the hepatitis relapses after therapy. In this study we investigated the efficacy of serum hepatitis C virus (HCV) RNA monitoring during IFN therapy to predict the outcome of a biochemical end-of-treatment (ETR) response. Eighty patients with chronic hepatitis C received leucocyte (natural) IFN-alpha (13 patients) or recombinant IFN-alpha2a (67 patients). Antiviral therapy was given for 12 months to 43 (53.7%) responders and this group was analysed further. During follow-up, 15 relapsed and 28 showed a sustained response (median follow-up 50 months, range 39-67 months). Viraemia was monitored at baseline, and at months 1, 3, 6, 9 and 12 of treatment, by nested polymerase chain reaction (PCR) (sensitivity 10-100 copies ml-1). A combination of positive nested PCR and HCV RNA values at the 3rd and 6th months of treatment was 100% predictive of relapse (sensitivity, 66.6%; specificity, 100%). A combination of negative nested PCR and HCV RNA values at the 1st and 3rd months of treatment was 100% predictive of sustained response (sensitivity, 39.3%; specificity, 100%). In conclusion, serum HCV RNA monitoring is an appropriate and reliable tool for predicting early outcome of the biochemical ETR response after IFN discontinuation. This could be useful in the modulation of therapeutic management of chronic hepatitis C.