Prothrombin Complex Concentrate (PCC) is indicated for the acute reversal of oral anticoagulation therapy. To compare the efficacy of a "standard" dosage of 20 ml PCC equivalent to about 500 IU factor IX (group A), and an "individualized" dosage based on a target-INR of 2.1 or 1.5, the initial-INR and the patient's body weight (group B), we performed an open, prospective, randomized, controlled trial. The in vivo response and in vivo recovery of factor II, VII, IX and X in these patients on oral anticoagulation was determined. Ninety three patients (group A: 47; group B: 46) with major bleedings or admitted for urgent (surgical) interventions were enrolled. PCC and Vitamin K (10 mg) were administered intravenously. We evaluated the effect of treatment by the decrease of INR and the clinical outcome. The number of patients reaching the target-INR 15 min after the dosage of PCC was significantly higher in the group treated with an "individualized" dosage, compared to the group treated with a standard dose, (89% versus 43%; p<0.001). So, we conclude that for the acute reversal of oral anticoagulant therapy, an "individualized" dosage regimen of PCC based on the target-INR, the initial-INR, and body weight of the patient, is significantly more effective in reaching the target-INR than a "standard" dosage. The in vivo response and in vivo recovery found in this study was higher then in patients with isolated factor deficiencies. This suggests that the pharmacokinetics in patients on oral anticoagulants may be different.