Genetic changes between codons 2209 and 2248 of NS5A of genotype 1b hepatitis C virus (HCV-1b) have been reported to be associated with the sensitivity to interferon-α (IFN-α). The present study was performed to analyze such relationship in Korean patients with chronic hepatitis C and HCV-1b (n=19),including 12 chronic hepatitis C patients treated with IFN-α, 3 chronic hepatitis C patients without treatment as controls, and 4 patients with hepatocellular carcinoma (HCC). Two serum samples, before and after the treatment, were analyzed for the mutations by reverse transcription-polymerase chain reaction, cloning and sequencing. The mutations were identified in 32% (6/19), including five intermediate type (1-3 mutations) and one mutant type (4 or more). In 12 patients treated with IFN-α, the number of amino acid substitutions in NS5A_2209-2248 was not associated with outcome of the treatment.