The intensity of anticoagulant treatment seems to be the most important predisposing factor for bleeding and the excessive effect of oral anticoagulants should be reversed. We evaluated the response of patients under oral anticoagulation for thromboembolism prevention to oral or endovenous vitamin K administration. Sixty-one patients were studied retrospectively, and received oral or endovenous vitamin K ministration. Anticoagulation was withdrawn and all patients were reassessed one day after vitamin K administration. For analysis, they were separated according to INR: Group A: n=20, INR 6 to 8 and 0.5mg endovenous vitamin K; Group B: n=24, INR 6 to 8 and 2.5mg oral vitamin K; Group C: n=7, INR 10 to 25 and 0.5mg endovenous vitamin K; Group D: n=10,1NR 10 to 25 and 2.5mg oral vitamin K. After treatment INR was below therapeutic range in 15% of group A and in 4% of group B. All other patients of groups A and B achieved therapeutic range on the first day of treatment. In group C, 57% of the patients were above therapeutic range on the first day and again received 0.5mg of endovenous vitamin K. On the second day all patients were within the therapeutic range. In group D, 30% of the patients had to repeat oral ministration of vitamin K in order to achieve the therapeutic range on the second day of treatment. None of the patients presented bleeding complications. Table 1 demonstrates median and standard deviation of INR in the four groups. The quick reversal (one day) obtained by the withdrawal of the drug and the ministration of oral vitamin K, without INR below the therapeutic range or bleeding complications, indicates this easy protocol for the treatment of overdoses of oral anticoagulation. GROUP A GROUP B GROUP C GROUP D N 20 24 7 10 day0 INR-median(± SD) 5.9(0.35) 6.0(0.44) 17.2(6.7) 15.8 (6.1) day1 lNR-median(± SD) 2.9(1.06) 2.9 (0.88) 7.0 (5.1) 5.6 (24).12SUPPL. 181