In chronic hepatitis C virus (HCV) infection, genotypes other than genotype 1b of HCV (HCV-1b) and low serum HCV-RNA levels are known to be associated with favorable outcome of interferon alfa (IFN-α) therapy. In addition, we recently reported a close correlation between the number of mutations in amino acid sequences 2209 to 2248 of the nonstructual protein 5A gene (NS5A2209-2248) of HCV-1b and the response to IFN-α. In the present study, we analyzed these viral factors in relation to the efficacy to IFN-β, another type I IFN. The pretreatment sera of 40 patients treated with IFN-β intravenously at 6 MU daily for 42 days were studied. HCV genotypes, serum HCV-RNA levels, and the amino acid sequence of NS5A2209-2248 in HCV-1b were determined. A sustained complete response to IFN therapy occurred in none of the ten patients with the wild-type HCV-1b who had an NS5A2209-2248 sequence identical to the prototype HCV-1b and in none of the six patients with the intermediate-type HCV-1b that had 1 mutation. In contrast, complete responses occurred in the following: 4 of 6 patients with the mutant-type HCV-1b that had five to ten mutations; 6 of 13 patients with genotype 2a of HCV (HCV-2a); and 2 of 5 patients with genotype 2b of HCV (HCV-2b). Among patients with the mutant-type HCV-1b or genotype 2 of HCV (HCV-2) the rate of complete response was significantly higher (12 of 24 vs. 0 of 16 patients, P < .001) and HCV-RNA levels were significantly lower (4.5 [4.0-6.5] vs. 6 [4.5-6.5] log copies/mL, median [range]; P < .001) compared with patients with the wild-or the intermediate-type HCV-1b. Patients with the mutant-type HCV-1b or HCV-2 whose HCV-RNA levels were lower than 6 log copies/mL had a complete response rate of 75% (12 of 16 patients) in contrast to 0% (0 of 24 patients) of the others (P < .001). These results indicate that the mutant-type HCV-1b or HCV-2 are sensitive to IFN-β as well as IFN-α. In conclusion, the determination of HCV genotypes, NS5A2209-2248 of HCV-1b and serum HCV-RNA levels may facilitate the selection of patients with a high likelihood of response to IFN-β.