Two types of Alzheimer neurofibrillary tangles may be found in the hippocampus in senile dementia of the Alzheimer type. Besides classical flame-shaped intraneuronal tangles, there are less compact tangles representing extracellular remnants of destroyed neurons with neurofibrillary change. Strong immunoreactivity for glial fibrillary acidic protein (GFA) was found in the second type of tangles, which was due to penetration of fine processes of fibrous astrocytes into bundles of paired helical filaments (PHF). PHF appear to be a strong stimulus for astrocytic reaction when they are not segregated from the neuropil by the neuronal cell membrane.