We recently developed a genome-length hepatitis C virus (HCV) RNA replication system (OR6) with luciferase as a reporter. The OR6 assay system has enabled prompt and precise quantification of HCV RNA replication. Pegylated interferon (IFN) and ribavirin combination therapy is the world standard for chronic hepatitis C, but its effectiveness is limited to about 55% of patients. Newer therapeutic approaches are needed. In the present study, we used the OR6 assay system to evaluate the anti-HCV activity of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors, called statins, and their effects in combination with IFN-α. Five types of statins (atorvastatin, fluvastatin, lovastatin, pravastatin, and simvastatin) were examined for their anti-HCV activities. Fluvastatin exhibited the strongest anti-HCV activity (IC50: 0.9 μmol/L), whereas atorvastatin and simvastatin showed moderate inhibitory effects. However, lovastatin, reported recently as an inhibitor of HCV replication, was shown to exhibit the weakest anti-HCV activity. The anti-HCV activities of statins were reversed by the addition of mevalonate or geranylgeraniol. Surprisingly, however, pravastatin exhibited no anti-HCV activity, although it worked as an inhibitor for HMG-CoA reductase. The combination of IFN and the statins (except for pravastatin) exhibited strong inhibitory effects on HCV RNA replication. In combination with IFN, fluvastatin also exhibited a synergistic inhibitory effect. In conclusion, statins, especially fluvastatin, could be potentially useful as new anti-HCV reagents in combination with IFN. (HEPATOLOGY 2006;44:117–125.)