We report 2 cases of 'de novo' aneurysm formation in a vessel which appeared to be normal at a previous angiography. The first patient developed an anterior communicating artery aneurysm nine years after occlusion of the right internal carotid artery by Gianturco coils for the treatment of a giant intracavernous carotid aneurysm. In the second case a 'de novo' aneurysm of the internal angle A1-A2 segment of the left anterior cerebral artery developed 6 years after successful clipping of another aneurysm of the same location. De novo formation of an aneurysm in a vessel which was found to be normal in a previous angiographic study, may occur as result of hemodynamic changes, such as after internal carotid occlusion or in presence of an arteriovenous malformation or variations of the circle of Willis. However, definite hemodynamic changes may also be absent. We conclude that patients operated on for aneurysm clipping must be periodically explored by magnetic resonance angiography to evaluate the possibility of de novo appearance of another aneurysm.