The 78-year-old male underwent coronary angiography because of angina pectoris. He was revealed to have essential thrombocythemia with a platelet count of over 1,000,000/mm3. Essential thrombocythemia belongs to the group of chronic myeloproliferative disorders. It displays both thrombogenic and bleeding tendency due to the increased platelet count, as well as to dysfunction. CABG was performed using the left internal thoracic artery and the right gastroepiploic artery. Hydroxycarbamide was taken to regulate the platelet count before surgery. There was no difficulty with hemostasis during surgery. Warfarin and hydroxycarbamide were used as anticoagulant therapies after surgery. Postoperative CAG demonstrated both grafts to be patients. The patient remained in good condition until he died suddenly on the 159th postoperative day. The cause of death was not clear because no autopsy was carried out. The death may have been associated with a thromboembolism, acute graft thrombosis or cerebral infarction, or pulmonary embolism. This patient did not take antiplatelet drugs because the platelet count and prothrombin time was well controlled. Nonetheless, an antiplatelet agent might reduce the risk of thromboembolism in such patients. It is suggested that meticulous anticoagulation therapy must be important for a patient with essential thrombocythemia, especially in the postoperative period.