A 36-year-old woman presented with sudden onset of a right-sided headache that awoke her from sleep. She had no episodes of trauma or abuse. She was initially able to speak, but fell into a coma within an hour. The right pupil was dilated, with slow pupillary reflexes to light on both sides, and she showed left hemiparalysis. Computed tomography scan showed a right acute epidural hematoma, approximately 4 cm in thickness, and there were no findings of trauma such as skin wounds, subcutaneous hematomas, or skull fractures. In the emergency room, decompression of intracranial pressure by one burr hole was performed, and her dilated right pupil improved to normal size. She was then moved to the operating room, and hematoma removal was performed by craniotomy. Her blood pressure trended downward despite rapid blood transfusion and vasopressor therapy. There were no abnormal findings apparent intraoperatively, except for oozing from the whole surface of the dura mater and epidural space. Her consciousness improved postoperatively, and her left hemiparalysis improved within a few days. No causative diseases, risk factors, or vascular abnormalities were found on laboratory and radiological surveys. Two months postoperatively, the bone flap was removed because of infection. Eight months postoperatively, a cranioplasty using artificial skull was performed, and her postoperative course was uneventful. One year after the initial surgery, she has no neurological deficits, and there has been no recurrence of epidural hematoma.