Warfarin is metabolized by cytochrome CYP2C9 and its pharmacokinetic properties depend on structural polymorphisms of CYP2C9 gene. We studied frequencies of allele variants of CYP2C9 gene and associations of individual reaction to warfarin intake with genotype of CYP2C9 gene. Population frequencies of CYP2C9x1, CYP2C9x2, CYP2C9x3 alleles of CYP2C9 gene in St-Petersburg were 82.66, 11.11, and 6.32%, respectively. Carriers of CYP2C9x2 and CYP2C9x3 alleles more rapidly achieved therapeutic levels of hypocoagulation and required significantly lower weekly doses of warfarin.