Infection with the hepatitis C virus may result in chronic liver disease for which no effective therapy is now available. We studied the effects of recombinant human interferon alfa in a prospective, randomized, double-blind, placebo-controlled trial in patients with well-documented chronic hepatitis C. Forty-one patients were enrolled in the trial, 37 of whom were later found to have antibody to hepatitis C virus. Twenty-one patients received interferon alfa (2 million units) subcutaneously three times weekly for six months, and 20 received placebo. The mean serum aminotransferase levels and the histologic features of the liver improved significantly in the patients treated with interferon but not in the patients given placebo. Ten patients treated with interferon (48 percent) had a complete response, defined as a decline in mean serum aminotransferase levels to the normal range during therapy; three others had a decrease in mean aminotransferase levels of more than 50 percent. After treatment ended, however, serum aminotransferases usually returned to pretreatment levels; 6 to 12 months after the discontinuation of interferon therapy, only two patients (10 percent) still had normal values. We conclude that interferon alfa therapy is beneficial in reducing disease activity in chronic hepatitis C; however, the beneficial responses are often transient.