The authors describe the case of a 19-day-old infant with a ruptured intracranial aneurysm. She had suddenly become lethargic after drinking milk. Cerebral angiography demonstrated a large aneurysm arising from the right middle cerebral artery. Because the aneurysm was fusiform, the parent artery was clipped. Histological examination of a specimen taken from the aneurysmal dome showed an organized thrombus with deposition of a calcium-like substance in the wall, which appeared on histochemical examination to be pseudolime. The postoperative course was uneventful and she was discharged without neurological deficits.