The densities of senile plaques (SP) and neurofibrillary tangles (NFT) were studied in the cerebral cortex in nine autopsy cases of Alzheimer's disease (AD) and seven cases of senile dementia of Alzheimer type (SDAT). The number of SP and NFT were counted in 14 selected areas of the cerebral cortex using methenamine‐silver and Gallyas‐Braak stains, respectively. The ratio of cortical atrophy in each AD or SDAT case to the mean value in normal age‐matched cases was estimated, and the results were corrected for cortical atrophy using this ratio. The SP density in AD after correction was significantly higher than that in SDAT in the superior and inferior frontal, rectal, anterior cingulate, superior temporal and suprapar‐ietal cortices. The NFT density in AD was also significantly higher in all of these areas and also in the medial temporo‐occipital cortex. The significant difference between AD and SDAT for NFT density after correction was greater than that for SP density in the superior and inferior frontal, anterior cingulate and superior temporal cortices. These results suggest that the higher densities in AD of SP and NFT in these areas are useful in distinguishing AD from SDAT. The NFT density was a more useful indicator than the SP density in differentiating between the two conditions. The greater density in AD of SP and NFT in the cerebral neocortex may reflect the focal clinical symptoms, frequently observed in AD. The quantitative difference between AD and SDAT in the two changes observed in this study was not due only to the difference in the duration of illness, because the SDAT cases with long survival showed significantly lower NFT density than AD cases with similar disease duration and brain weight. It is suggested that degeneration process in AD progresses more rapidly than tha