The article presents the results of the research of gene СYP2С9 polymorphism in 100 patients with atrial fibrillation. The existence of three genotypes of СYP2С9 gene was shown: CC homozygote (wild type) – in 83 patients, heterozygous CT – in 13 patients and TT homozygote (mutant) in 4 patients. The relationship between genotype and preventive dose of warfarin was established, i.e. the dosage was lower in the carriers of the mutant allele, while the frequency of hypercoagulability (INR more then 3) was above. The above findings suggest the necessity of carrying out genotype research in order to identify patients needing more careful selection of the maintenance dose of warfarin and more attentive laboratory monitoring.