Interferon-alpha therapy is of proven efficacy in chronic hepatitis C, but it is not universally effective and may be associated with intolerable side effects. Ribavirin is a nucleoside analog with a broad spectrum of antiviral action. We conducted an uncontrolled pilot study of ribavirin therapy in 13 patients with chronic hepatitis C. Ribavirin was given for 6 mo, in a dose that was increased, at 2-mo intervals, from 600 mg to 1,000 mg to 1,200 mg/day. Serum ALT levels gradually decreased in all 13 treated patients; the mean percentage of decrease was 67% (from 210 U/L [range = 109 to 593] to 63 U/L [range = 22 to 108 U/L]; p = 0.0006) after 6 mo of treatment. Serum aminotransferase levels fell to the normal range in four patients (31%). In the 3 to 6 mo after cessation of ribavirin therapy, serum aminotransferase activities gradually rose to near pretreatment levels in all but one patient. Therapy was associated with a significant decrease in the geometric mean titer of hepatitis C virus RNA in serum (1:1,981 vs. 1:199; p less than 0.02) although no patients lost hepatitis C virus RNA from serum during therapy. No significant improvement was seen in liver histological appearance. Ribavirin therapy resulted in mild, reversible hemolysis; no patient exhibited symptomatic anemia. These findings suggest that ribavirin has a beneficial effect in patients with chronic hepatitis C, although further studies are needed to determine how ribavirin is best used.