To determine whether cerebral blood flow (CBF) progressively decreases as dementia progresses, we studied 133Xe washout at two or three different times in 23 demented patients. All 15 patients with Alzheimer's and 5 of 8 patients with vascular dementias had more rapid declines of CBF than did nondemented age-matched controls. Mean rate of change in Alzheimer's disease was —0.60 per month, compared with +0.29 per month in four controls tested twice. Regression analysis of CBF with age in 30 controls tested once showed a decline of CBF with aging at a rate of —0.013 per month. The rates of CBF decline in dementia were significantly correlated with rates of change of behavioral score measuring functional impairment.