Single photon emission tomography with Hm PAO Tc99m was used in 27 patients with probable Alzheimer's disease, 9 patients with multi-infarct, dementia, and 11 healthy volunteers. The data were quantified in 4 regions of interest (using the basal ganglia as an internal standard): left and right prefrontal areas (Fg and Fd), and left and right parietal areas (Pg and Pd). Pg and Pd indices were lower in patients with Alzheimer's disease than in patients with multi-infarct dementia, and healthy volunteers (p less than 0.05). The ratio (Fd + Pd)/(Fg + Pg) was lower in patients with dressing apraxia (p less than 0.02), anosognosia (p less than 0.001), spatial agnosia (p less than 0.01), ideomotor apraxia (p less than 0.02), ideatory apraxia (p less than 0.01), and constructive apraxia (p less than 0.01). No correlation could be established between this ratio and the severity of aphasia, or between the indices and mini mental state scores. With Pg and Pd indices lower than 0.90, sensitivity for Alzheimer's disease was 70 p. 100 and specificity, 75 p. 100.