Paired helical filament (PHF)/tau immunoreactive dystrophic neurites are a common pathological feature in the brain of patients with Alzheimer's disease. Recent studies suggest that swollen neurofilament-immunoreactive neurites are also present in senile plaques. In the present study, we investigated whether PHF/tau-positive dystrophic neurites are located in all subtypes of plaques and whether swollen neurofilament-immunoreactive neurites are hyper-phosphorylated, using a battery of antibodies to PHF/tau, neurofilament, and beta-amyloid protein. PHF/tau-positive dystrophic neurites were present in and around nearly all subtypes of plaques, including small amyloid deposits, diffuse plaques, and perivascular plaques in the hippocampal formation of Alzheimer brain. The earlier changes were detectable with AT8 antibody and later changes with PHF-1 antibody. Plaque-associated PHF/tau-positive dystrophic neurites were rare or absent in the hippocampal formation of normal aged brain. Swollen neurofilament-positive neurites appeared to be hyper-phosphorylated in Alzheimer's disease and to a lesser degree in aged control brains. Neurites that contained hyper-phosphorylated tau as well as neurofilament were strongly argentophilic because both populations of dystrophic neurites stained with silver stains. Swollen neurofilament-positive plaque-associated neurites were often present in the absence of PHF/tau-positive plaque-associated dystrophic neurites. These data suggest that PHF/tau-positive dystrophic neurites are a common component of all subtypes of plaques in Alzheimer brain and neurofilament protein in swollen neurites, like tau protein, is hyper-phosphorylated. Hyper-phosphorylated neurofilaments in plaque-associated neurites may represent one of the earliest cytoskeletal changes in vulnerable neurons in Alzheimer's disease and aged control brains.