Interferon (IFN) was administered intermittently for 6 months to the patients with non-A non-B chronic hepatitis (CHNANB), and the effectiveness of the treatment for improving the hepatic function was evaluated. Of 26 patients with CHNANB, 16 received intermittent IFN therapy (IFN group), and 10 were treated by conventional therapies without IFN (non-IFN group). All patients were observed for 1 year. IFN was administered once a day at 3 MU in principle (1 MU in some patients and 6 MU in 1 patient) daily for 1 week immediately after the beginning of the therapy and 3 times a week for the subsequent 6 months at the outpatient clinic. The patients were followed up for at least 6 months after completion of the treatment. In the IFN group, the serum GPT level normalized in 11 (68.8%) of the 16 patients 1 year after the beginning of the treatment. In these 11 patients (normalized group), HCV-RNA was negative or became negative in 3 of the 6 patients in whom the serum HCV-RNA could be examined. Histological grades of inflammation in the liver were also markedly alleviated in the normalized group. The hepatic function did not normalize in any of the 10 patients in the non-IFN group. These findings indicate that IFN therapy is useful for CHNANB.