An 88-year-old woman presented with acute subdural hematoma (ASDH) which showed rapid resolution on computed tomography (CT) and magnetic resonance (MR) imaging. She was transferred to our hospital after falling out of bed. On admission, she was comatose with Japan Coma Scale score of 200 and Glasgow Coma Scale score of E1V1M2. Brain CT showed a thick left frontotemporal ASDH. Conservative treatment consisted of 200 ml of glycerol administered intravenously twice a day, and maintenance in the approximately 20 degree head-up position to reduce intracranial pressure. Three days later, her consciousness recovered to Japan Coma Scale score of 30 and Glasgow Coma Scale score of E2V4M5. CT showed obvious reduction of the hematoma without brain or scalp swelling. Spinal MR imaging detected no redistribution of hematoma to the spine. The present case illustrates that rapid spontaneous reduction of ASDH may occur by redistribution of hematoma, mainly to the supratentorial subdural space because of brain atrophy.