Regional cerebral blood flow (rCBF) was measured in 23 patients with organic dementia (17 patients with Alzheimer's disease and six with low-pressure hydrocephalus [LPH]). The flow of gray matter was significantly reduced, especially in occipito-parieto-temporal regions, as well as frontally in some patients. During activation with psychologic tests, reading, etc., the flow augmentation in the association areas in the demented patients was not as marked as in nondemented controls. Instead, in some patients, a flow diminution was recorded in the same regions (intellectual steal). Organic dementia appears to be accompanied by a low activity in the association cortex and a reduced ability to activate these regions during mental effort.