Recently, we evaluated the clinical significance of a new assay, the semi-automated Cobas Amplicor HCV Monitor test v 2.0 (Roche, Cobas-HIGH-RANGE assay), for quantifying serum hepatitis C virus RNA throughout a wider range than the traditional assay, Amplicor GT HCV Monitor test v 2.0 (GT-ORIGINAL assay). We compared the results of the Cobas-HIGH-RANGE assay with results of the GT-ORIGINAL assay. This study was conducted on serum from 91 patients with chronic hepatitis C at the Gastroenterological Center, Yokohama City University Medical Center. The percent coefficient of variation (CV) for the within-run and between-run reproducibility of the Cobas-HIGH-RANGE assay ranged from 0.7 to 2.3% and from 1.3 to 2.0%, respectively. The Cobas-HIGH-RANGE assay exhibited a linear range extending from 5 to 5000 KIU/ml. The values for all 17 samples determined as less than 0.5 KIU/ml by the GT-ORIGINAL assay were also determined as less than 5 KIU/ml by the Cobas-HIGH-RANGE assay. For the 48 samples with values of 0.5 to 850 KIU/ml determined by the GT-ORIGINAL assay, the values obtained by these two assay methods were significantly correlated (r2 = 0.9117, y = 1.0667x-0.0801, p < 0.001), but the values of HCV-RNA determined by the Cobas-HIGH-RANGE assay were significantly (p < 0.05) higher than those determined by the GT-ORIGINAL assay. Consequently, these data indicate that the HIGH-RANGE assay is a useful alternative assay method for measurement of HCV-RNA instead of the ORIGINAL assay, and that the HIGH-RANGE assay could be a useful tool for monitoring the efficacy of antiviral treatment.