A 61-year-old male with hypertension presented with sudden onset of headache and nausea due to subarachnoid hemorrhage (SAH). He had two siblings with history of SAH due to ruptured intracranial aneurysms. Right carotid angiography on admission showed an anterior communicating artery aneurysm. At that time, the extracranial arteries were not examined. The aneurysm was clipped with no complications. A pulsating mass was palpable in the abdomen 37 days after the onset. Ultrasonography and computed tomography showed an abdominal aortic aneurysm with intraluminal thrombus, measuring 8 x 9 x 8 cm. Normal pressure hydrocephalus had already developed. The patient underwent elective abdominal aortic aneurysm resection before ventriculoperitoneal shunting. After shunting, he recovered fully. The present case indicates that unpredictable sudden enlargement of associated abdominal aortic aneurysm is possible in patients with ruptured intracranial aneurysms.