Raw scores for cerebral blood flow (CBF), determined by xenon 133 inhalation, and for Wechsler Adult Intelligence Scale (WAIS) were measured in five groups of 12 subjects each: young normals, aged normals, and patients with cerebrovascular disease without dementia, dementia with cerebrovascular disease, and degenerative brain disease. Important differences were present in the raw data according to age and sex. When these were adjusted, a quadratic model revealed a highly significant (p less than 0.0001, r = 0.86) correlation between CBF and measured intelligence score. We interpret these findings to indicate that in dementing illnesses the WAIS raw score reflects the severity of the brain disorder, regardless of cause, and that CBF is reduced as a function of the severity rather than the cause of the abnormality.