Preparation of the patient for operation depends on complex assessment of the patient’s condition and surgical intervention. On one side the risk of bleeding linked to intervention and the rate of increased risk by anticoagulation therapy is assessed. In other side the basal risk of thrombotic complications is evaluated due to basic disease of the patient and the risk of postoperative venous thromboembolism due to the patient’s age, characteristics of surgical procedure and accessory risk factors. Procedures with the low risk of bleeding (tooth extraction, small skin operations, cataract operations, endoscopy of the digestive tract, mucous biopsy including) might be performed within continual anticoagulation therapy with values of INR 2.0 - 3.0. In other operations warfarin has to be withdrawn about 5 days before the operation. After decrease of INR below the low level of therapeutic range (except the patients with the low basal risk of thrombosis) it is necessary to begin with administration of a low molecular heparin (LMWH) in a dose corresponding to the stage of basal risk of thrombosis. After the operation LMWH is administered in a prophylactic dose, the full dose of LMWH is administered, if indicated, after 48 - 72 hours, if no complications are present. In preparation for an acute operation the effectiveness of warfarin has to be eliminated by administration of a small dose of vitamin K. In case of an urgent surgical procedure earlier than in 12 hours the concentrate of prothrobin complex factors has to be administered.