The morphological findings of 2 basilar artery giant fusiform aneurysms are presented. In one case (a 63-year-old man) the aneurysm was accidentally found at autopsy. Its wall was mainly formed by fibrous tissue without a smooth muscle layer and presented fragmented but still recognizable elastic lamina. In the media there were small well-formed bony spicules. In the other case (a 59-year-old man) the aneurysm had broken causing subarachnoid hemorrhage. The wall showed a marked reduction of smooth muscle cells and thinning and fragmentation of elastic lamina. A second sacciform aneurysm was present at the basilar tip. The review of the literature and the morphological findings of the 2 cases, characterized by abnormality of the portion of the basilar artery not directly involved in the aneurysm wall, consisting of a diffuse deficit of the tunica media and lamina elastica, might suggest that the fusiform aspect of the aneurysm may be the result of the degenerative effect of atherosclerosis on a cogenital, structural or dysmetabolic, or acquired, inflammatory, weakening of the arterial wall.