Cytochrome P4502C9(CYP2C9) is largely responsible for terminating anticoagulant effect by hydroxylation of S-warfarin to inactive metabolites. Mutations in the CYP2C9 gene result in the expression of allelic variants, CYP2C9*2 and CYP2C9*3 with reduced enzyme activity compared to wild type CYP2C9*1. The aim of this study was to assess relationship between requirement of warfarin dose and polymorphism in CYP2C9 in Korean population. Patients on warfarin therapy for longer than 1 year were included from July 1999 to December 2000 and categorized as one of four groups; regular dose non-bleeding, regular dose bleeding, low dose non-bleeding and low dose bleeding. Low dose was defined as less than 10 mg/week for 3 consecutive monthly follow-ups. Bleeding complications included minor and major bleedings. Blood samples were processed for DNA extraction, genotyping and sequencing to detect polymorphism in CYP2C9. Demographic data, warfarin dose per week, prothrombin time (INR), indications and co-morbid diseases were assessed for each group. Total 90 patients on warfarin were evaluated; The low dose group has taken warfarin 7.6 +/- 1.7 mg/week, which was significantly lower than 31.4 +/- 0.9 mg/week in the regular dose group (p<0.0001). The measured INR in the low dose group was similar to that of the regular dose group (2.3 +/- 0.7 vs. 2.3 +/- 0.6, p=0.9). Even though there was a higher possibility of CYP2C9 variation in the low dose group, no polymorphism in CYP2C9 was detected. All patients were homozygous C416 in exon 3 for CYP2C9*2 and A1061 in exon 7 for CYP2C9*3. The DNA sequencing data confirmed the homozygous C416 and A1061 alleles. In conclusion, polymorphism in CYP2C9 is not a critical factor for assessing warfarin dose requirement and risk of bleeding complications in a Korean population.