Chronic hepatitis C (non-A, non-B hepatitis) is a common and often progressive viral liver disease. To assess the efficacy of therapy with the antiviral agent interferon alfa, we randomly assigned 166 patients with chronic hepatitis C to treatment with either 3 million or 1 million units of recombinant interferon alfa three times weekly for 24 weeks, or to no treatment. The probability of normalization or near normalization of the serum alanine aminotransferase levels after six months of interferon therapy was 46 percent in patients treated with 3 million units of interferon (P less than 0.001) and 28 percent in those treated with 1 million units (P less than 0.02), but only 8 percent in untreated patients. The serum alanine aminotransferase level became completely normal in 22 of the 26 patients (85 percent) who responded to treatment with 3 million units of interferon and 9 of the 16 patients (56 percent) who responded to treatment with 1 million units. The patients who received 3 million units of interferon had histologic improvement because of the regression of lobular and periportal inflammation. Relapse within six months after the completion of treatment occurred in 51 percent of the patients treated with 3 million units of interferon and 44 percent of those treated with 1 million units. We conclude that a 24-week course of interferon therapy is effective in controlling disease activity in many patients with hepatitis C, although relapse after the cessation of treatment is common.