Chronic hepatitis C is an insidious disease associated with significant morbidity and mortality. Currently, the only approved therapies for chronic hepatitis C are the α interferons. Consensus interferon (CIFN) is a nonnatural, synthetic, recombinant type I interferon derived by assigning the most commonly observed amino acid in each position of several α interferon nonallelic subtypes to generate a consensus sequence. The efficacy and safety of CIFN in the treatment of chronic hepatitis C were assessed in two large phase 3 trials. The first trial was a multicenter, randomized, double-blind, controlled study of 704 patients who were treated with one of two doses of CIFN (3 microg and 9 microg) or interferon alfa-2b (3 million units [MU]) weekly for 24 weeks and then observed for an additional 24 weeks. Treatment with CIFN at a dose of 9 microg was safe and effective, with serum alanine aminotransferase (ALT) and hepatitis C virus (HCV) RNA sustained response rates of 20% and 12%, respectively. Responses to 3 MU interferon alfa-2b were comparable to 9 microg CIFN. The response rates were lower in the 3-microg CIFN cohort. At the end of the treatment and posttreatment observation periods, an undetectable serum HCV RNA was a better predictor of a normal ALT than the converse. Serum samples from early time points were available for HCV RNA quantitation from 27 of the 28 patients who experienced a sustained response with 9 microg CIFN. Of these, 13 patients (48%) had undetectable HCV RNA at 2 weeks, 21 patients (78%) at 4 weeks, and 26 patients (96%) at 12 weeks. CIFN (9 microg) induced a significantly greater reduction in the mean serum HCV RNA concentration than interferon alfa-2b during treatment (P < .01). In patients with high viral titers (> or = 4.75 × 10(6) copies/mL), the HCV RNA sustained response rate in patients treated with CIFN (9 microg) and interferon alfa-2b was 7% and 0%, respectively (P = .03). In patients infected with HCV genotype 1, the HCV RNA end-of- treatment (24% vs. 15%; P = .04) and sustained (8% vs. 4%; P = not significant) response rates were greater in patients treated with CIFN (9 microg) than with interferon alfa-2b (3 MU). In a subsequent multicenter trial, a higher dose of CIFN (15 microg) was reinstituted in patients who either had relapsed or were nonresponders to prior CIFN or interferon alfa-2b therapy. Patients were randomized to receive 24 or 48 weeks of retreatment followed by 24 weeks of observation. Patients who had relapsed after prior interferon therapy were more likely to have a serum HCV RNA end-of-retreatment and sustained response than patients who were nonresponders to prior interferon therapy. After patients from the 3-microg CIFN cohort were excluded, the HCV RNA sustained response rates were 28% in relapsers and 5% in nonresponders, respectively, in the 24-week retreatment cohort and 58% and 13%, respectively, in the 48-week retreatment cohort. The administration of 9 or 15 microg CIFN was well tolerated, and the adverse effects were similar to those for interferon alfa-2b. These data demonstrate that CIFN at a dose of 9 microg is effective initial therapy for patients with chronic hepatitis C, and that retreatment with a higher CIFN dose of 15 microg for 48 weeks provides meaningful responses in both relapsers and nonresponders.