Periodic synchronous discharges were consistently observed in a 68-year-old man with a four-year history of progressive dementia. Pathological examination revealed diffuse atrophy of the brain with remarkable dilatation of the temporal horns. Histologically there were severe changes of Alzheimer's disease and diffuse Lewy bodies (LB), either typical or less distinctive ones without clear halos. Cerebellar LB were also observed in the cerebellum. Four types of abnormal intraneuronal filamentous structures were observed: neurofibrillary tangles, accumulated neurofilaments, thick linear structures studded with ribosome-like dense granules, and fuzzy, thin filaments similar to, but generally wider than, a neurofilament.