Objective To investigate the effects of chronic hepatitis B virus (HBV) infection on human hepatic cytochrome P450 2C9 (CYP2C9).Methods Liver tissue samples and blood samples were obtained from 10 patients with chronic HBV infeetion and 10 healthy controls.CYP2C9 genotypes were determined by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) method.The activity of CYP2C9 was detected utilizing high performance liquid chromatography (HPLC).The expressions of CYP2C9 mRNA and protein were determined by reverse transcriptase-polymerase chain reaction (RT-PCR) and Western-blotting.The data were analyzed by t test.Results All the liver samples showed CYP2C9 wild-type (*1*1),while CYP2C9 (*2) and CYP2C9 (*3) were not detected.The maximum velocity (Vmax) of CYP2C9 in patients chronic HBV infection and healthy controls were (263.5±66.4) μmol/L and(284.6±85.9) μmol/L,respectively (t=0.614,P=0.5471).The expression of CYP2C9 mRNA in patients with chronic HBV infection (0.39±0.28) was significantly lower than that of healthy controls (0.65±0.13) (t=2.628,P=0.0171).Accordingly,the protein expression in patients with chronic HBV infection (0.26±0.13) was lower than that of healthy controls (0.60±0.19) (t=4.688,P=0.000 2).Conclusion The expressions of CYP2C9 mRNA and protein are decreased in chronic HBV infection which may down-regulate the enzyme activity.