Hepatitis C virus (HCV) isolates from 12 patients with chronic hepatitis C underwent the sequence analysis. Among the 12 patients, 9 obtained a complete response (CR), 2 partial response (PR) and 1 non--response (NR) after the treatment with IFN-alpha. The results showed that only single amino acid (aa) substitution in iIFN sensitivity determining region (ISDR) (aa 2213 and aa 2218) in 6 cases with CR was observed, while 3 cases of CR and all PR and NR cases had no aa mutation of ISDR. It is suggested that the ISDR in HCV NS5A was yet not identified by our patients.