Resistance to vitamin K antagonists is a rare phenomenon. In general, it is the result of poor patient compliance, malabsorption, a diet rich in vitamin K, or the use of enzyme inducers. Occasionally, the diagnosis of resistance is made when plasma levels are high. The authors report a case of hereditary resistance to vitamin K antagonists in a 43 year old man admitted for atrial fibrillation. The precise prevalence of this anomaly is unknown. However, the existence should be known to clinicians who often use this important family of drugs.