Submitted for the study were 116 autopsy brains, from 65 non-demented people, 24 patients with dementia of Alzheimer type (DAT) and 27 patients with vascular dementia, aged between 50 s and 100 s. Formalin-fixed, paraffin-embedded coronal sections of the brains at the level of the lateral geniculate body were immunohistochemically stained with the avidin-biotin-peroxidase complex procedure, using the anti-bodies to tau protein purified from human brains (anti-tau) as the primary antibodies. Alzheimer neurofibrillary tangles (NFTs) which were specifically and selectively stained by anti-tau were semiquantitatively counted in the areas of the hippocampus, parahippocampal gyrus and lateral occipitotemporal gyrus. The results were as follows: 1) In non-demented subjects, NFTs in the hippocampus and parahippocampal gyrus were scanty in the 50 s: they increased markedly after 60 years until 90 as the patients' age increased; they tended to decrease over 90 years. In contrast, NFTs in the lateral occipitotemporal gyrus remained none or scanty, always less than 10/mm2 field, throughout the ages between 50 s and 100 s. 2) In DAT cases, NFTs in the hippocampus and parahippocampal gyrus were numerous in all cases at any ages. NFTs in the lateral occipitotemporal gyrus were also many, and always more than 10/mm2 in all cases except a few ones over 80 years of age. The numbers of NFTs of the three areas were significantly higher in DAT cases than in non-demented subjects. 3) In most cases of vascular dementia, the density and distribution pattern of NFTs were essentially similar to those of non-demented subjects.(ABSTRACT TRUNCATED AT 250 WORDS)