We report the case of a young healthy woman who presented an early overanticoagulation when receiving acenocoumarol for a first thromboembolic episode. The patient had none of the risk factors known to influence the response to the coumarinic derivative except that she carried the rare *3 allelic variant of the cytochrome P450 CYP2C9 in a homozygous status. This case illustrates the role of the *3 polymorphism of the cytochrome P450 CYP2C9 as an independent risk factor modulating the sensitivity of patients to the anticoagulant effect of acenocoumarol. The usefulness of CYP2C9 genotyping before starting coumarinic treatments is discussed.