The efficacy of interferon therapy for chronic hepatitis C were evaluated by the changes of serum RNA of the hepatitis C virus (HCV) by the polymerase chain reaction. Before the treatment, HCV-RNA was detected in all of the 17 patients. 5 patients given IFN for 4 weeks, 12 cases given IFN by the 2 weeks/on 2 weeks off schedule (4 cases given 4 cycles and 8 given 6 cycles). Immediately after the treatment ended, the HCV-RNA was not detected in 12 (71%) cases. Five cases were persistently positive for HCV-RNA and showed continuously abnormal level of ALT. In 7 of 12 cases who became negative for HCV-RNA just after the treatment ended, HCV-RNA was detected again at first and 12th month after therapy. The ALT level were continuously abnormal in 4 of these 7 cases during and after the treatment, and became normal transiently in the other 3. In the 5 cases who became persistently negative (at least 1 year), ALT level decreased to normal or near-normal during follow up for more than one year. According to our results, we classified the therapeutic efficacy of IFN therapy for chronic hepatitis C. Type I response is complete response. In this type, HCV-RNA became negative persistently and ALT became normal or near-normal for at least 12 months after therapy. Type II is partial response. In this type, HCV-RNA became negative transiently and ALT level was transiently normal or poor. Type III is ineffective. In this type, HCV-RNA did not become negative and the effect for ALT was poor.(ABSTRACT TRUNCATED AT 250 WORDS)