Compared to either drug alone, therapy with the combination of ribavirin and interferon-alpha leads to improved rates of response in patients with chronic hepatitis C. Side effects often mandate downward dose adjustment or cessation of therapy, and the optimal dose of ribavirin has not been established. The aim of this study was to learn whether 600 mg ribavirin per day would prove as efficacious as 1,000-1,200 mg/day when combined with interferon (3 million units thrice weekly) for therapy of patients previously treated with standard interferon who had failed to respond or who had relapsed. We enrolled 69 patients with chronic hepatitis C and compensated liver disease: 45 were men, 65 were Caucasian, 48 were infected with genotype 1 hepatitis C virus. By random assignment, 35 received 600 mg ribavirin/day (group A), whereas the other 34 received 1,000 mg (< or = 75 kg body wt) or 1,200 mg/day (>75 kg body wt) (group B). At baseline, the two groups were well matched for demographic and laboratory features. In both groups, mean serum levels of alanine aminotransferase (ALT) and hepatitis C viral (HCV) RNA levels fell promptly and remained significantly lower than baseline throughout 24 weeks of therapy. There was no significant difference in mean levels of ALT or HCV RNA during therapy or at the end of follow-up (24 weeks after cessation of therapy). At the end of 24 weeks of posttherapy follow-up, 12 patients in each group had undetectable HCV RNA in serum, whereas 11 (31%) in group A and 9 (26.5%) in group B had normal serum ALT levels. The lower doses of ribavirin (group A) were tolerated better. In conclusion, in previous nonresponders or relapsers to interferon done, combination therapy with interferon-alpha2b (3 MU thrice weekly) + 600 mg ribavirin/day is tolerated better and is as effective as interferon plus higher (standard) doses of ribavirin (1,000-1,200 mg/day).