Cerebral perfusion patterns in 18 cases with vascular dementia of Binswanger type (VDBT) (8 moderate and 10 severe cases) were compared with 25 cases with senile dementia of Alzheimer type (SDAT) (16 moderate and 9 severe cases) and 14 controls by single photon emission computed tomography using N-isopropyl-p-123I iodoamphetamine (IMP) as a tracer. The cerebral: cerebellar IMP uptake ratio (%) (CCR) was used as a measured of relative cerebral perfusion. The CCRs were about 85-90% in all areas in controls. Moderate VDBT patients showed a remarkable decrease of CCRs in the basal grey region (thalamus and basal ganglia) (right 79%, left 77%) and in the frontal area (right 79%, left 80%) (P less than 0.01). In severe VDBT patients a significant decrease of the CCR was noted in all regions (P less than 0.01). The decrease of mean CCRs in the hemispheres was significantly correlated with the severity of disease determined by psychometric testing. Patients with SDAT showed a significant decrease of the CCR in the parietal (right 71%, left 74%) and right temporal (78%) areas in the moderate stage (P less than 0.01), and further progression of dementia was associated with low perfusion areas extending to the the frontal areas (78%, P less than 0.01). These differences in the perfusion patterns and their changes with progression of the illnesses may be reflected in characteristic clinical features.