A case is described in which chronic bilateral obstruction of the internal carotid arteries by giant cell arteritis of their intracavernous sinus portions had produced progressive dementia (Case 1). It is suggested that the most important factor in the production of cerebral symptoms with bilateral carotid obstruction is the presence of defective anastomoses in the circle of Willis, as was so in Case 1. Mental changes are present in about 30% of cases but they are non-specific. Partial bilateral carotid obstruction may occur with senile dementia (Case 2) where it may play no role. Surgical treatment may be possible if the obstruction is proximal in the internal carotid artery, and the circle of Willis is intact.