HCV is an RNA-virus which contains a positive-strand RNA genome. The RT-PCR method is used to detect the HCV-RNA in plasma. Quantification of HCV-RNA is difficult, but it can now be done by a competitive RT-PCR method developed recently. Genotypes of HCV can be classified into at least six basic groups, genotype I-IV can be distinguished by RT-PCR with type-specific primers. We report here that the prevalence of type-I, II, III, IV, and mixed type (II + IV) are 1.0%, 90.3%, 3.2%, 3.2%, and 2.2%, respectively. The DNA-probe method was recently developed and is less sensitive than the RT-PCR method, but simple and handy to detect HCV-RNA. Patients with chronic active hepatitis C received 3MU (9MU) of natural-interferon alpha (recombinant-interferon alpha 2a) daily for two weeks followed by three times a week for 22 weeks. Although the relationship between genotype of HCV and response to interferon-alpha therapy is unclear, the titer of HCV-RNA was significantly lower in long-term responders (ALT level remained within the normal range during the 12 weeks after the end of therapy) and short-term responders (ALT level rose again during the 12 weeks after the end of therapy) than that in nonresponders (ALT level was not normalized in spite of interferon-alpha therapy).