The gradual intraneuronal accumulation of an insoluble fibrous material which partly consists of abnormally phosphorylated tau protein (neurofibrillary change) represents an important neuropathological hallmark of Alzheimer's disease. Neurofibrillary tangles and neuropil threads formed from this material develop in only a few types of cortical pyramidal cells. The first changes are seen in the entorhinal cortex. The destructive process then spreads into the hippocampal formation and eventually encroaches upon the isocortex. This sequence of events permits the distinction of six stages with a progressive increase in the severity of cortical destruction. The entorhinal region serves as an important interface between the isocortex and hippocampus. This interface function is markedly impaired due to the early deterioration of the entorhinal cortex. Severe entorhinal involvement is considered to represent the morphological counterpart of clinically incipient Alzheimer's disease. Similar changes are found in mentally impaired individuals suffering from Parkinson's disease or progressive supranuclear palsy.