We measured CSF alpha-melanocyte stimulating hormone-like immunoreactivity (alpha-MSH-LI) in 35 patients with dementia of the Alzheimer type (DAT) and in 27 healthy control subjects. Mean alpha-MSH-LI concentration was significantly decreased in DAT patients as compared with age-matched controls. However, when the DAT patients were analyzed according to age at onset of dementia or presence of extrapyramidal signs, alpha-MSH-LI concentrations remained significantly lower than in controls only in DAT patients with late onset of dementia (greater than 65 years of age). No correlation was found between alpha-MSH levels and degree of mental impairment. A significant negative correlation was found between CSF concentrations of alpha-MSH and homovanillic acid in the group of all DAT patients (p less than 0.001). These results suggest that hypothalamic neurons which produce pro-opiomelanocortin-related peptides may be involved in Alzheimer's disease.