ORAL ANTICOAGULATION with a vitamin K (phytonadione) antagonist has always been a problematic therapy. The risk of serious bleeding is substantial (2%-3% yearly) and does not decrease over time. In fact, as patients undergoing long-term therapy age, the bleeding risk increases. The therapeutic effect of oral anticoagulation falls in a narrow range between increased risk of thrombosis when the international normalized ratio (INR) is too low and increased bleeding risk when it is too high.