Multi-infarct dementia (MID) and dementia of the Alzheimer type (DAT) were compared with regard to ventricular size and leuko-araiosis (LA) on computed tomographic scans. Ninety-seven percent of patients with MID and 55.5% of patients with DAT had evidence of LA. The severity of LA was scored with a 0 through 4 rating system, and LA was found to be significantly more severe in patients with MID than in patients with DAT. Patients with MID, but not those with DAT, exhibited correlations between enlargement of the third and lateral ventricles and severity of cognitive impairment.