Neurofibrillary tangle staging was compared in the nucleus basalis and cerebral cortex of Alzheimer's disease patients with and without Lewy body disease. In pure Alzheimer's disease, cholinergic nucleus basalis cell number, as determined from counts in serial forebrain sections, was 22-60% of control mean, with the majority of residual cells containing tangles. A comparison between control cell number and the combined number of tangles plus tangle-free neurons in pure Alzheimer's disease suggests that the majority of nucleus basalis neurons were lost through neurofibrillary degeneration. The staging of neurofibrillary degeneration in the nucleus basalis was discordant with cortical changes as some controls had more extensive tangle formation in the nucleus basalis than in the cerebral cortex. Patients having both Alzheimer's disease and Lewy body pathology had few or no tangles in the nucleus basalis despite greater loss of neurons than purely demented patients. The presence of concomitant pathology had a greater effect on nucleus basalis tangle burden than did cortical disease stage, suggesting dichotomous disease processes in the cerebral cortex and forebrain.