Warfarin is the major oral anticoagulant drug used in Norway. The effect is monitored by measuring the patient's prothrombin time expressed as international normalized ratio (INR). The dose necessary to achieve a therapeutic INR varies. A few patients require extremely high doses, and hereditary warfarin resistance has been reported.The present paper describes two brothers with reduced sensitivity to warfarin. A review of the literature is given.Both patients achieved therapeutic levels of INR, but required daily doses of 25-35 mg warfarin.Hereditary warfarin resistance is a rare cause of reduced sensitivity to warfarin. It is not associated with increased thromboembolic risk.