The microtubule-associated protein tau, a major antigenic component of paired helical filaments, has been demonstrated in neurofibrillary tangles and in neurites of senile plaques. With optimal fixation and histochemical methods, we show the normal axonal location of tau protein in human cerebral cortex and the striking alterations of tau distribution that affect the cortical neuropil in Alzheimer's disease. Normally, cortical tau-immunoreactive fiber bundles form a pattern resembling that seen with myelin stains. The prominence of white matter staining suggests that tau may be especially enriched in projection systems. Alzheimer's disease causes massive axonal disruption and the dislocation of tau protein from its usual axonal domain into neuronal cell bodies, dendrites, and presynaptic regions. The normal pattern of axonal staining in cortex is disrupted and white matter staining is reduced. Prominent abnormal tau-immunoreactive neuropil fibers are densely present even in cortical regions without classical neurofibrillary tangle and senile plaque formation. The striking neuropil abnormalities, revealed by the aberrant localization of tau protein, are likely to contribute to neuronal dysfunction in Alzheimer's disease.