Although multiple cerebral aneurysms ae well recognized, a new aneurysm has only rarely been documented after successful treatment for an aneurysm elsewhere. In our consecutive series of 986 patients with intracranial saccular arterial aneurysm collected from 1975 to 1990, nine patients who had previously unverified (hence, de novo) intracranial aneurysms and ruptures at intervals of 4 to 7.5 years after clipping of an initial aneurysm are presented here. All nine had undergone successful treatment of a previous aneurysm; preoperative and postoperative angiography showed not only successful clipping of the first aneurysm but also no incidence of multiple aneurysms. These patients had suffered from hypertension before their second admission. Seven of the nine patients were treated surgically. All patients had experienced angiographical or symptomatic vasospasm after the first subarachnoid hemorrhage. In the second admission however, seven patients who underwent the surgery for a new aneurysm suffered from no vasospasm in spite of the prominent second subarachnoid hemorrhage. Two of the nine patients died of primary brain damage due to the hemorrhage and underwent necropsy. A histological study of a new aneurysm demonstrated the same findings as that of a usual saccular aneurysm. This clinical study of our patients suggests that it is important to control blood pressure for protection against a new aneurysm formation.