Rapid reduction of excessively high international normalized ratio (INR) values into the therapeutic range is necessary in patients who must be maintained on oral anticoagulants but who present with an overdose. We prospectively studied the effect of observation alone versus escalating low doses of intravenous vitamin K on 23 patients presenting with no overt evidence of hemorrhage and INR values in excess of 10. Two of six patients observed without intervention developed spontaneous hemorrhage. Only one of the observed patients and none of 4 patients given 100-200 µg-vitamin K had INR values less than 5.0 by 24 hours. Three of the four patients receiving 500 µg of vitamin K had INR values less than 5.0, with two of these being less than 3.5 within the first 24 hours. Nine patients receiving 1000 µg of vitamin K had INR values between 1.7 and 3.5 by 24 hours. In none of these nine patients did the INR subsequently normalize, and there were no difficulties re-establishing anticoagulant therapy. No adverse effects were noted in any of the patients receiving intravenous vitamin K. We recommend 1000 µg of intravenous vitamin K as a safe and effective means of rapidly reversing excessively anticoagulated patients presenting with INR values greater than 10 units.