Factors predictive of the response to interferon in patients with chronic hepatitis C remain to be identified. In this study, we investigated factors predictive of the short-term response, defined as a return to normal alanine aminotransferase activity after treatment, and the long-term response defined as normal alanine aminotransferase activity 1 year after completing treatment, in 75 patients with chronic hepatitis C virus treated with recombinant alpha interferon (either 6 MU x 3/week for 3 months then 3 MU x 3/week for 3 months (n = 27) or 3 MU x 3/week for 6 months (n = 48)). At the end of treatment, 42 patients (56%) had normal alanine aminotransferase activity ("responders") and 33 (44%) had high alanine aminotransferase activity ("non-responders"). Twenty (48%) of the 42 responders had normal alanine aminotransferase activity 1 year after treatment ("sustained responders"), while 22 (52%) had high alanine aminotransferase activity ("transient responders"). The dosage of interferon was not predictive of the short-term and the long-term response to treatment. The responders differed significantly from the non-responders in terms of age, i.v. drug abuse, aspartate aminotransferase, gammaglutamyltranspeptidase and alkaline phosphatase activities, bilirubinemia, serum bile acid concentrations, prothrombin time, platelet count, ferritinemia, hyaluronic acid levels, positivity for the antibody to 5.1.1 of the recombinant immunoblot assay band and the histological fibrosis score. The following parameters were independently correlated with the short-term response in a multivariate analysis: gammaglutamyltranspeptidase activity, serum bile acid concentrations and positivity for the antibody to 5.1.1 of the recombinant immunoblot assay band.(ABSTRACT TRUNCATED AT 250 WORDS)