Five cases of Alzheimer's disease are reported and the literature is reviewed.All patients showed profound intellectual deterioration, motor unrest, affective changes and a disorder of speech. The age of onset ranged from 44 to 64. The duration of the illness varied from 2½ to 26 years. In one case the disease came to a standstill for more than two years. Certain observations suggested that exogenous factors could accelerate the progress of the disease. An increase of muscle tonus occurred in four cases. One patient presented a clinical picture resembling that observed in certain cases of Pick's lobar atrophy. Apraxia and visual agnosia were noted in one case and cerebellar signs in another. Symptoms suggestive of vegetative involvement were not uncommon.The disorder of speech did not conform to any of the classical types of aphasia. It was looked on as a manifestation of a general lowering of function to a more primitive level caused by diffuse changes in the brain.The chief histologic changes were nonspecific cell lesions, diffuse cell losses, senile plaques and neurofibril lesions of the Alzheimer type. These alterations showed a considerable degree of parallelism in the cerebral cortex but not elsewhere. The glial response was usually active. Hyaline-like vascular lesions were found in all cases.All parts of the cerebral cortex were affected. In most instances the parietal lobes, the cornu ammonis and the temporal lobes presented the severest damage. The observations indicated that the pathologic process could occasionally be accentuated enough in certain regions to produce focal symptoms. As a rule there was no close correspondence between the areas of greatest involvement and the individual cyto-architectural fields. The changes were usually most pronounced in the third layer of the cerebral cortex. The basal ganglia frequently showed considerable involvement. In one case marked alterations were observed in the cerebellum.The differences and resemblances between Alzheimer's disease and senile dementia and the various theories regarding the origin of the former are discussed.A consideration of all the facts lends support to the view that the clinicopathologic picture of Alzheimer's disease occurs in a rather heterogeneous group, the individual members of which may owe their origin to different etiologic agents. The possibility cannot be excluded that Alzheimer's disease may in most instances be caused by the same factors operative in senile dementia. The observations presented here tend to suggest that a close relationship between the two conditions is perhaps less frequent than has hitherto been supposed.