Abstract: Aims/Background: Hepatitis C virus (HCV) infection is frequently diagnosed by detection of antibody to the HCV (anti-HCV). Recently, a method for detection of HCV core protein, “Imucheck F-HCV Ag Core Kokusai”, has been developed. In this study, we evaluate the utility of this method. Methods: HCV core protein levels in sera were determined using this following method; anti-HCV titres were measured by particle agglutination (PA) test and then quantitative HCV-RNA values were investigated using a competitive reverse transcription-poly-merase chain reaction (RT-PCR) test. Results: The HCV core protein was detected only in anti-HCV-positive sera. Of 490 anti-HCV-positive sera, 130 (26.5%) were positive by this method. Of 144 anti-HCV-positive/HCV-RNA-positive sera, 130 (90.3%) were positive by it. A significant correlation between the HCV core protein levels and quantitative HCV-RNA values was recognized (n=110, r=0.86, p<0.01). A significant correlation between the HCV core protein levels and alanine aminotransferase titres was also observed (n=67, r=0.72, p<0.05). All 71 patients with chronic active hepatitis, cirrhosis and hepatocellular carcinoma were positive with this method, whereas 18 of 32 patients with chronic inactive hepatitis were positive. Twenty-three patients with chronic active hepatitis were treated with interferon-α. During therapy, some patients showed a negative conversion of HCV core protein. One (7.7%) of 13 patients with HCV genotype 1 and 5 (62.8%) of 8 patients with genotype 2 remained negative for 6 months after the therapy. Conclusion: This method may be useful for quantitative evaluation of HCV viraemia in anti-HCV-positive patients.