Two cases of spontaneous dissecting aneurysm of the cervical internal carotid artery are reported, and 10 cases recorded in the literature are reviewed. The apparent rarity of this condition may well be due to failure of recognition. The dissecting aneurysm in our first case was associated with significant atherosclerotic changes in the internal carotid artery; and in our second case, bilateral internal carotid artery involvement was present, with a dissecting aneurysm on the right side and complete occlusion of the artery on the left side. Histologic changes of fibromuscular dysplasia were found in the right internal carotid artery of this case. The importance of prompt angiographic demonstration of this condition is evident, since the chance of effective surgical intervention is markedly decreased after the development of complete occlusion and fixed neurologic deficit.