Large neurons in the neostriatum of patients with Alzheimer's disease (AD) and progressive supranuclear palsy (PSP) were investigated topographically, histologically and ultrastructurally. The number of large neurons whose nuclear area is greater than 101 microns2 was uniformly decreased in the neostriatum in PSP, but the decrease of these neurons in AD appeared to be more marked in the nucleus accumbens. Most of the remaining large neurons in both diseases contained neurofibrillary tangles (NFTs). In addition, some of the small neurons in PSP were positive for tau-immunostaining. Curly fibers were frequently observed in AD, but were absent in PSP. Ultrastructurally, NFTs in AD were composed mainly of paired helical filaments, whereas those in PSP contained straight tubules.