Relatively few patients with chronic hepatitis C treated with standard doses of interferon-alpha 2b (3 million units per week for 24 weeks) have a sustained response. Our aim was to evaluate whether higher doses of interferon would improve this rate of response. Twenty-four patients with chronic hepatitis C who had failed to respond to (N = 21) or had relapsed after (N = 3) an initial course of standard interferon therapy were randomized to 15 million units (N = 13) or 22.5-30 million units per week (N = 11) for 24 weeks. Five of 13 subjects given 15 million units per week and 3/11 of subjects given 22.5-30 million units per week had complete normalization of serum alanine aminotransferase levels during therapy. Five patients (24% who had not responded to standard interferon had a complete response to high-dose interferon during therapy. Only one patient had a sustained response, with normal serum alanine aminotransferase 24 weeks after stopping interferon. Six patients were withdrawn before completing treatment, five in the 22.5-30 million unit per week group. We conclude that higher doses of interferon ameliorate the severity of hepatitis in patients who failed to respond to or relapsed after standard interferon therapy, but are unlikely to produce a sustained response. High-dose therapy is associated with an increase in side effects.