Regional cerebral blood flow measured in patients with dementias (N = 60) using xenon Xe 133 inhalation was compared with measurements in health volunteers (N = 70). Volunteers were age-matched (N = 15); another group was younger. In normal aging, there is progressive, diffuse reduction of weight and flow of gray matter (F1) but not of white matter. Therefore, age-matched control subjects are necessary in studies of dementia. In Alzheimer's disease (AD), F1 shows bilateral and symmetrical reduction. The F1 reduction correlated with atrophy estimated by computerized tomography, and duration and severity of dementia. In multi-infarct dementia (MID), bilateral hemispheric F1 was patchily reduced. Cerebral vasodilator response to 5 percent carbon dioxide inhalation was reduced in patients with MID but was normal in patients with AD. Patients with Wernicke-Korsakoff's dementia showed normal values. Patients with dementia due to multiple sclerosis showed significant F1 reduction compared with normal subjects. Standard behavioral activation in all patients with moderate to severe dementia failed to produce normal F1 increases.