Six hours after the surgery, she walked, uneventfully. Twelve hours after the surgery, she walked a second time and developed nausea, cold sweats, and hypotension (systolic blood pressure, 60 mmHg). Infusion of Ringer's lactate solution was started, but her heart rate increased to 140-150 · min Ϫ1 . Blood gas analysis revealed severe hypoxia with mild hypercapnia (Pa O 2 , 37 mmHg; Pa CO 2 , 47.4 mmHg). As pulmonary embolism was suspected, oxygen inhalation (5 l · min Ϫ1 ) was begun, and 5000 units of heparin was given. Thrombus was identified in the pulmonary artery by computed tomography (CT) with contrast. Echocardiography revealed thrombus in the right atrium and right ventricle.Fifteen thousand units of heparin and 120 000 units of urokinase were given per day. Warfarin was started 1 week after the onset day. The thrombus was not identified in the pulmonary artery on CT with contrast after 7 days of urokinase therapy. She was discharged from the hospital on warfarin and cilostazol 22 days after the surgery, without sequelae.The present patient had no previous history of thrombus, was 54 years old, and had a platelet count of 787 000 · mm 3Ϫ1 . Although a bone-marrow biopsy was not performed, she was clinically considered to have low-risk ET, and clinical observation alone was performed. The duration of surgery and the amount of blood loss were both minimal. Accordingly, we employed only elastic bandages on the lower extremities during anesthesia, and used no other means of prevention of DVT. However, pulmonary embolism occurred, probably at the time of the second walk by the patient 12 h after the surgery. The findings in this patient and a recent case report [2] suggest that the prevention of DVT by intermittent pneumatic compression and/or anticoagulant prophylaxis, such as low-dose heparin administration, should be considered in patients with low-risk ET, even for minor surgery.