The accumulation of aluminum in the cerebral cortex has been implicated as a factor in the pathogenesis of Alzheimer type senile dementia (SD) and in the dialysis dementia found in patients with chronic renal failure on long-term intermittent hemodialysis treatment. In animal studies, parathyroid hormone (PTH) produces increased absorption of aluminum from the gastrointestinal tract and elevations of aluminum in the cerebral cortex. It has been proposed that PTH elevations may increase tissue aluminum loads in patients with senile dementia. The present study was undertaken to investigate the status of circulating PTH in patients with SD and age/sex matched controls. No significant differences were found between these groups. Elevated PTH (when it did occur) seemed to be related to the degree of renal impairment rather than dementia. Differences in the distribution of aluminum in patients with dialysis dementia and SD are discussed.