Results of a standardized histochemical and immunocytochemical analysis of the brains of 14 nondemented elderly humans for whom prospective neurological and neuropsychological data had been collected for 3 to 8 years before death suggested that nondemented elderly humans fall into two pathological subgroups that are not clinically distinguishable. One was associated with moderate to marked cerebral amyloid deposition ("pathological aging"), while the other had either minimal or no amyloid deposition ("normal aging"). Neocortical and hippocampal neurofibrillary degeneration was either completely absent or of very limited degree in both subgroups. Both subgroups had ubiquitin-immunoreactive dystrophic neurites in the cerebral cortex and granular degeneration of myelin in white matter. These ubiquitin-immunoreactive structures seem to be a universal and invariant manifestation of brain aging, but the same cannot be said for amyloid deposition and neurofibrillary degeneration. Pathological aging might be preclinical Alzheimer's disease, but it currently cannot be distinguished from normal aging by even sensitive neuropsychological measures. These findings provide strong support for the hypothesis that cerebral amyloid deposition is not necessarily associated with clinically apparent cognitive dysfunction and that additional factors, such as neuronal or synaptic loss or widespread cytoskeletal aberrations, are necessary for dementia in AD.