MRI scans of 27 patients with probable Alzheimer's disease (mean age 68.2 years), 31 patients with vascular dementia (mean age 69.9 years) and 18 normal controls (mean age 66.3 years) were compared to evaluate possible distinguishing parenchymal abnormalities among these groups. Atrophy was quantitated by subjective rating, linear and volumetric measurements. A number of findings were significantly more common in vascular dementia than in the other subsets. These included (1) basal ganglionic/thalamic hyperintense foci, (2) thromboembolic infarctions, (3) confluent white matter and (4) irregular periventricular hyperintensities. Signal abnormalities on intermediate T2-weighted scans in the uncal-hippocampal or insular cortex were frequently and almost exclusively noted in Alzheimer's disease. Moderate and severe cortical and ventricular atrophy and a third ventricular to intracranial width ratio larger than 7% were good discriminators between demented groups and normally aging controls. Selective atrophy measurements, however, failed to separate dementia syndromes. These results suggest that MRI has the potential to increase the accuracy of the clinical diagnosis of Alzheimer's disease and vascular dementia.