We have made a histological study of the cerebral cortex and hippocampus from 59 consecutively autopsied subjects, aged 51-102 years, prospectively assessed for the presence and extent (or absence) of dementia. Counts were obtained of the numbers of argyrophilic plaques and neurofibrillary tangles in each area examined. In demented patients without evidence of cerebrovascular disease, a highly significant correlation was found between the presence and severity of dementia and the number of neurofibrillary tangles in the cerebral cortex. A significant but much weaker and inconstant correlation was obtained between the presence and severity of dementia and the number of argyrophilic plaques present in the cortex. The temporal lobe cortex and hippocampus were the areas most severely affected by the increased neurofibrillary tangle formation in senile dementia due to Alzheimer's disease.