Quantitative measurement of serum hepatitis C virus(HCV) RNA is important in predicting and monitoring interferon(IFN) therapy. We compared the sensitivity of HCV RNA measurement of different HCV genotypes between two available assays, Roche Monitor 1.0(v1.0) and Roche Monitor 2.0(v2.0). We also evaluated serum level of HCV RNA as the predictors of a long-term response to IFN therapy by distinguishing the complete responders(CR), partial responders(PR) and non-responders(NR) for IFN therapy. We quantified the serum HCV RNA levels in 151 patients and determined the genotypes; 96(64%) with genotype 1b(1b), 42(28%) with genotype 2a(2a), and 6(4%) was not identified. The relationship between the genotype and effects of IFN treatment was as follows: 23CR(1b:11, 2a:9), 15PR(1b:8, 2a:7), 20NR(1b:14, 2a:6). The RNA levels of 2a measured by v2.0 were significantly higher than those by v1.0(p < 0.05), although no significant difference was found in 1b between two assays. By using v2.0, when the cut-off level was set at 200 x 10(3) IU/ml before IFN therapy, CR was discriminated from PR, NR with a predicting efficiency of 88%. HCV RNA levels before IFN therapy were significantly lower in patients who became HCV RNA negative within 2 weeks than in patients who did at 4 weeks or longer. These results suggest that v2.0 is more sensitive and accurate than v1.0 for the quantification of 2a. Using v2.0 assay, it was shown that low viral titre at pretreatment and loss of viraemia within 2 weeks after treatment might be important markers for a long-term response to IFN therapy, irrespective of viral genotype. The v2.0 assay was found to be more useful in predicting effectiveness of IFN therapy.