Interferon is commonly used for treatment of type C hepatitis, but the effects are variable and many factors may be responsible. Hepatitis C virus (HCV) can be classified into 4 types, PT, K1, K2a and K2b. Therefore, the responses to interferon treatment in patients with the different HCV genotypes were analyzed. Twenty-four patients with type C hepatitis were treated with 3 to 10 million units of various types of interferon for more than 8 weeks. HCV-RNA encoding the NS5 region (HCV-NS5) was positive in these 24 patients, 16 of which were classified with the K1 type and 8 with the K2 type of HCV. In all patients except for 2, HCV-NS5 became negative within 3 weeks of treatment without relation to the HCV genotypes. Serum alanine aminotransferase levels were normalized in 7 out of 8 patients in the K2 group and in 4 out of 16 patients in the K1 group at the end of 8 weeks. At the 24th week, ALT levels were normalized in 5 out of 6 patients in the K2 group, and in one out of 9 patients in the K1 group. The percentage of patients exhibiting a good response was significantly higher in the K2 group than in the K1 group at both observation periods. During the post-treatment periods, relapse following complete response was found in 3 patients in the K2 group and in one patient in the K1 group. The final effects of interferon were significantly better in the K2 group than in the K1 group.(ABSTRACT TRUNCATED AT 250 WORDS)