The cerebrospinal fluid neuropeptide Y level was measured by radioimmunoassay in 20 patients with probable Alzheimer's disease and in 19 controls. The mean level was lower in patients (69.5 +/- 36.7 pg/ml) than in controls (103 +/- 21.8 pg/ml; p less than 0.001). Patients with a disease duration of greater than 2 years had cerebrospinal fluid neuropeptide Y levels lower than those with shorter disease duration (p less than 0.02). These results suggest that neuropeptide Y containing cells may be involved in Alzheimer's disease. No correlation was found between neuropeptide Y levels and degree of cognitive impairment or age at disease onset.