Aluminium is neurotoxic and results in the proliferation of 100 A diameter filaments in the cytoplasm of certain neurons. The aluminium concentration for 7 normal human brains was 1-9 +/- 0-7 SD mug/g dry weight (n = 208 samples). The aluminium content of 585 areas sampled in 10 post-mortem cases of Alzheimer's disease ranging in age from 50 to 88 years, in which the diagnosis was based on the specific histological appearances, revealed an elevated aluminium content in some regions. A range of 0-4 - 107-0 mug/g was encountered and 28 per cent of all regions sampled had concentrations in excess of 4 mug/g. Five of 6 cerebral biopsies from patients with Alzheimer's disease also had elevated aluminium content. In 2 additional Alzheimer's brains with neurofibrillary degeneration restricted to certain anatomical areas, elevated aluminium content was found to be associated with neurofibrillary degeneration and not with senile plaques. Furthermore, elevated aluminium content in multiple cortical regions was not found in 2 vascular dementias of the elderly. While the cytotoxic concentration for human neurons is unknown, the cytotoxic concentration for cat's cerebral neurons appears to lie between 4 and 6 mug/g dry weight.