Cerebral atrophy was measured by comparing brain volume and cranial capacity at necropsy on 20 severely demented patients aged 64--92 years and 18 non-demented controls of similar age. The volumes of cerebral cortex and white matter and of the lobes of the cerebral hemispheres were found by point-counting morphometry. Patients with Alzheimer type dementia aged less than 80 years mainly showed pathological cerebral atrophy with global loss of cerebral tissue (P less than 0.001) whereas over 80 years of age they generally showed selective atrophy of temporal cortex (P less than 0.005). The findings support the view that disease processes, not exaggerated age change, underlie primary neuronal dementia of Alzheimer type.