The study included 25 patients at high risk of thromboembolic complications. All of them were treated with acenocoumarol for 6 months under control of the frequency of hemorrhage and episodes of severe hypocoagulation (a more than 3-fold rise in INR). All the patients underwent CYP2C9 and VKORC1 genotyping. It was shown that the presence of CYP2C9*2 and CYP2C9*3 alleles in the CYP2C9 locus and the AA genotype of the polymorphous G-1639(3673)A marker of the VKORC1 gene was not associated with the development of severe hypocoagulation episodes (p = 0.261--for CYP2C9, p = 0.616 and 0.361 for VKORC1 in the total group and a subgroup of patients having the CYP2C9*1/*1 genotype respectively and treated with acenocoumarol. The search for other genetic markers of efficacy and safety of this drug should be continued.