The intramuscular administration of interferon-beta (IFN-beta) at a dosage of 6 million units three times per week for 6 months has been evaluated in 90 patients included in a multicenter, randomized, controlled trial for the treatment of chronic hepatitis C. Transaminase levels were significantly reduced in IFN-beta-treated patients (p = 0.015) and were significantly lower with respect to those of the untreated controls (p = 0.040 at 6 months). Four treated (8%) and one untreated (2.5%) patients had normal transaminase values after 6 months. At study end (12 months), three quarters of the IFN-beta-treated patients had sustained transaminase normalization, whereas the untreated case had relapsed. Hepatitis C viremia was cleared in 6 (12%) treated patients but in none of the untreated controls (p = 0.058). Side effects of IFN-beta were infrequent (a mild flu-like syndrome in < 10%, asthenia in 16%, anorexia in 8%, headaches and weight loss in 8%, and hair loss in 4%). Leukocyte and platelet counts decreased during IFN-beta treatment, but no dose modifications were necessary. Such decreases were not statistically significant when compared with the levels in the untreated controls. Intramuscular IFN-beta at the dosage used has little efficacy in the treatment of chronic hepatitis C. Because of IFN-beta tolerance, higher doses and alternate routes of injection might prove beneficial for the treatment of this disease.