A 45-year-old patient with Down's syndrome was autopsied. The brain, weighing 800 g, was small in size, and serial sections revealed generalized gyral atrophy and ventricular dilatation. In the gray matter, there was diffuse neuronal degeneration characterized by numerous neurofibrillary tangles (NFTs), senile plaques and frequent amyloid angiopathy. Histochemical and electron microscopical analyses of these lesions showed no qualitative difference from those in Alzheimer's disease. A topographical study of NFTs showed that they were numerous in the limbic system and cerebral neocortex. Various numbers of NFTs were seen in the olfactory bulb, thalamus, medial geniculate body, innominate substance, putamen, caudate, pallidum, central gray, reticular formation, certain midline nuclei of the brainstem, substantia nigra, red nucleus and dorsal vagal nucleus. This distribution pattern was not different qualitatively from that in Alzheimer's disease, and such a similarity was especially evident in the olfactory bulb, where many tufted and mitral cells as well as anterior olfactory nucleus cells showed NFTs. These common features of brain pathology in Down's syndrome and Alzheimer's disease may be due to a specific gene defect in both diseases.