Traumatic intracranial aneurysms are rare. Because CT has largely replaced cerebral angiography in the examination of head-injured patients, the diagnosis of traumatic intracranial aneurysm was become much more difficult. Traumatic intracranial aneurysm should be suspected when a patient has an episode of intracranial hemorrhage after a head injury.A case of a “true” traumatic intracranial aneurysm is presented in this paper. A 9-year-old boy in a stuporous state was admitted on March 6, 1982 after a traffic accident. X-ray films demonstrated a rt. frontal linear fracture and a fracture of the rt. tibia.A CT scan showed subarachnoid hemorrhage mainly in the interhemispheric fissure. The patient regained consciousness after 3 days of conservative treatment. Twenty-four days after admission, during rehabilitation for a rt. leg fracture, he fell down and was hit in the frontal area. He rapidly deteriorated.A CT scan showed a large hematoma in the bifrontal lobes. An emergency craniectomy was done. During exposure, fresh arterial bleeding occurred, so an intraoperative carotid angiography was done. The angiogram showed a lt. anterior cerebral artery (A2) aneurysm. By reopening the craniectomy, the aneurysm, which was attached to the falx and was broad based, was clipped at the base. Postoperatively he gradually improved, but mild mental change remained. Judging from the operative findings, the aneurysm was probably a “true”traumatic aneurysm. Before the operation, we misjudged the source of the hematoma as being contusion from the second head injury.In a case of delayed or unusual intracranial hemorrhage after a head injury, cerebral angiography should be performed before surgical procedure.