In a study of senile degenerative lesions-including Alzheimer's neurofibrillary changes, senile plaques and amyloid angiopathy-the hippocampal area of the brain was examined by thioflavine T fluorescence microscopy in 146 consecutive autopsy patients over the age of 49. The incidence and quantity of neurofibrillary changes and senile plaques rose with age, and an approximate positive correlation in quantity was noted among the three kinds of degenerative change. The quantity of neurofibrillary lesions and senile plaques was significantly different between the demented and non-demented patients, but not between the severely and less severly demented patients. The cause of dementia was studied retrospectively, based on the extent of morphologic changes in the brain, thus classifying dementia into three types: degenerative, vascular, and mixed. Clinically, the mixed type resembled the vascular type with regard to major neurologic signs, and there was some similarity to the degenerative type with regard to mental features.