The light microscopical, immunohistochemical and ultrastructural aspects of eosinophilic bodies in the cerebral cortex from patients with Alzheimer's disease (AD) are described, based on a study of 16 cases of AD, 5 elderly non-demented controls and, as disease controls, 5 cases of Pick's disease, 9 with progressive supranuclear palsy, 5 with Creutzfeldt-Jakob disease and 1 with Binswanger's disease. At the light microscopy level, the bodies were clearly separated from the surrounding tissues and were mostly round or elliptic with a diameter of 5-30 microns and a central, intensely eosinophilic core. Ultrastructurally, they consisted of a central homogeneous electron-dense body (HDB), and filamentous structures (resembling either neurofilaments or paired helical filaments) or other small organelles in the periphery. Immunohistochemically, some of these bodies exhibited ring-shaped rims which were positive with antibodies against paired helical filaments, tau-2, phosphorylated neurofilaments and ubiquitin. The bodies were widely distributed throughout the cerebral cortex, but were not observed in the white matter. These bodies were thought to be compatible with one type of axonal dystrophy in the gracile nucleus (termed 'old' spheroid by Jellinger), and are here referred to as the HDB-type spheroid based on their ultrastructure. In this study HDB-type spheroids were found in high incidence in the AD cases, but only two HDB-type spheroids were seen in one case of Pick's disease, and none in any of the other cases of neurodegenerative diseases or in the elderly non-demented controls. It seems plausible that the incidence of HDB-type spheroids in the cerebral cortex might be related to a pathological process and not to a physiological ageing phenomenon, and might be characteristic of, but not unique to, AD.