Rapid reduction of a large acute subdural hematoma has been frequently reported. In my knowledge, however, it was rarely reported that rapid spontaneous reduction occurred in large volume of spontaneous intracerebral hematoma (sICH). We describe a patient with a rapid spontaneous decrease in the volume of a large hematoma. A 73-year-old man presented semi-comatose mentality. Initial brain computed tomography (CT) revealed the huge sICH. An emergency operation was planned, but was not performed due to the refusal of patient's family. Therefore, we decided to treat with conservative therapy. However, follow-up brain CT 16 hours after initial scan showed a remarkable reduction of previous sICH. The mechanism involving the spontaneous rapid decrease of the hematoma is presumed to occur through redistribution in brain atrophy, compression effect from the increased intracranial pressure and dilution through a wash out by the cerebrospinal fluid.