The authors report two patients with aneurysmal subarachnoid hemorrhage in whom carotid arteriography revealed aneurysms that had developed at previously normal locations and from infundibula during the years since initial angiography. Neither of these patients had congenital anomalies of the cerebral circulation and, apart from smoking in excess of 30 cigarettes a day, there were no common clinical features. The role of congenital and environmental factors in the formation of aneurysms and the enlargement of infundibula are reviewed.