A case of posttraumatic delayed cerebral arterial spasm is presented. A 71-year-old man was admitted to our hospital with head injury. Neurological examination on admission only revealed consciousness disturbance (Japan Coma Scale 30). CT scan 19 hours after the injury demonstrated a contusional hematoma in the right frontal lobe, faint subarachnoid hemorrhage in the left sylvian fissure and subdural hematoma in the interhemispheric fissure. His consciousness was disturbed on the 14th day. CT scan demonstrated a left subdural effusion, which was surgically evacuated. However, from the next day the patient developed left hemiparesis. Right carotid angiogram on the 17th day after the injury revealed multiple segmental arterial narrowing in the right anterior cerebral artery (ACA) and middle cerebral artery (MCA). We diagnosed a posttraumatic delayed cerebral arterial spasm. CT scan revealed low density areas in the right ACA and MCA territory. The pathogenesis of posttraumatic delayed arterial spasm is not yet well known. Now, four theories have been suggested as follows: (1) Subarachnoid hemorrhage, (2) Direct mechanical injury to the arterial wall, (3) Hypothalamus dysfunction, and (4) Disturbed autoregulation. In our case, three important factors are suggested. The first is direct injury to the artery, the second is cerebral contusion, and the third is subdural effusion.