Two hundred and thirty-four patients with chronic non-A, non-B hepatitis, 86% positive for anti-HCV by ELISA, were treated with recombinant interferon-alpha 2a or with natural (human-leukocytes-derived) interferon-alpha using different dosage and periods of administration. Interim analysis of follow-up data indicate that 65-70% of patients treated initially with 6 MU, thrice weekly, of recombinant interferon-alpha 2a achieved a complete biochemical response (normalization of alanine aminotransferase: ALT) during therapy compared to 56-58% of those treated with 3 MU, thrice weekly, of recombinant or natural interferon-alpha. A 12-month schedule of interferon administration appeared superior to a 6-month schedule in reducing the probability of reactivation of liver disease after therapy withdrawal, although further data are needed to confirm such a conclusion. The probability of response to interferon in terms of maintaining normal ALT after withdrawal did not appear to be influenced by sex, while it was significantly higher in patients aged below 45 years and in those without cirrhosis.