Previous studies demonstrated paired helical filament tau (PHF tau) in neuritic but not diffuse beta-amyloid (A beta) plaques in Alzheimer's disease (AD). Re-examination of amyloid deposits with antibodies to A beta and PHF tau by conventional and confocal microscopy using double label immunohistochemistry showed that PHF tau is a component of both diffuse and neuritic plaques in AD. Unlike controls, a dense network of PHF tau positive dystrophic neurites extended throughout the AD neocortex permeating nearly all neuritic and diffuse plaques. Thus, PHF tau-rich dystrophic neurites are common components of neuritic and diffuse plaques in AD neocortex.