Two cases of angiographically and surgically documented de novo intracranial aneurysm formation are reported. The first patient, a 32-year-old woman, developed two new aneurysms within a 6-month period, and the second patient, a 27-year-old woman, developed two new aneurysms within a 22-month period. In both patients, the new aneurysms were symptomatic, causing stroke and subarachnoid hemorrhage, respectively. The development of aneurysms de novo over such a short period of time has important implications for periodic imaging in patients considered to be at high risk for intracranial aneurysm formation.