An ANEURYSM of the internal carotid artery in the temporal bone is extremely rare in occurrence. Only seven previous reports were found in the literature.1-7 Biologic Characteristics It would appear that aneurysmal dilatations have a congenital factor in their development with a deficiency in the supportive bony canal wall and vessel architecture. One might well suspect that either trauma to the skull or an extensive middle ear infection may be contributory factors, but neither has been substantiated. On the other hand, the almost total absence of this aneurysm as a complication in fractures of the skull and extensive middle ear and mastoid infections is most impressive. Pathology This vascular phenomenon may appear as a true aneurysm with dilatation of the media and intima into a saccular compartment, or as a pseudo or false aneurysm with no vessel wall architecture present in the saccule and only a cavity containing a