Very few patients develop completely new intracranial aneurysms during long-term follow-up after successfully treated subarachnoid hemorrhages. Aneurysms appearing after therapeutic ligation of proximal major vessels or failed surgery and the growth of previously noticed infundibular widenings or small aneurysms must be excluded to find true de novo aneurysms. Twenty-nine true cases of de novo aneurysms were reported in the literature, and 13 additional cases of our own are described. The incidence of de novo aneurysm formation and rupture is 63 per 100,000 per year in patients known to have a subarachnoid hemorrhage. Young patients could benefit from long-term neuroradiological follow-up.