In acute hepatitis C, the efficacy of alpha interferon has not been definitively demonstrated. However, several small trials have suggested that interferon may decrease the chronicity rate of acute hepatitis C. In view of the high rate of chronic infection resulting from acute hepatitis C, it is warranted to treat patients during the acute phase of illness if they continue to have HCV RNA detectable in serum for 1 month after the onset of symptoms. The regimen of alpha interferon therapy should be 3 mu three times weekly for 24 weeks. In chronic hepatitis C, therapy with 3-5 mu of alpha interferon three times weekly for 24-48 weeks will induce a temporary remission in disease with loss of HCV RNA from serum, fall of aminotransferases into the normal range, and improvement in liver histology in 50% of patients, and a sustained remission persisting after therapy is stopped in 15% to 20% of patients. Younger patients with a short duration of disease, without cirrhosis, and with lower levels of HCV RNA in serum are the most likely to respond. Unfortunately, there are no completely reliable means of predicting which patients will derive long-term benefits from the use of interferon. Interferon remains the only approved therapy for chronic hepatitis C, although the low rate of sustained remissions and the incidence of side effects mandate a search for ways to improve the efficacy of interferon, as well as to find new, more potent agents for the treatment of this disease.