Magnetic resonance imaging (MRI) has revealed age-related changes in midbrain volume in normal subjects. Atrophy of the midbrain in patients with progressive supranuclear palsy (PSP) has been demonstrated using conventional brain MRI, and it was recently reported that some patients with vascular dementia also have midbrain atrophy. The aim of the present study was to investigate if the patients with subcortical ischemic vascular dementia (SIVD) have atrophic changes in the midbrain. MRIs of 23 SIVD patients, 18 probable PSP patients, and 96 controls were retrospectively analyzed. Differences in the distances between midbrain structures were compared across the patient groups and controls. We measured the anteroposterior diameter (AP), and the distance between the interpeduncular fossa and the aqueduct (IF–AQ), the aqueduct and posterior margin of the superior colliculi (AQ–SC), and the peduncular prominence and the interpeduncular fossa (PP–IF) of the midbrain. The AP diameter and IF–AQ were negatively correlated with age in normal controls (r = -0.21, p < 0.005 for AP; r = -0.14, p < 0.0001 for IF–AQ). In SIVD patients, the AP diameter and IF–AQ were both significantly smaller than in controls (p < 0.001). Changes in the midbrain found for SIVD patients were similar to those seen in PSP patients. Our results suggest that the midbrain decreases in size with normal aging, especially around the tegmental region. This change is more pronounced in patients with SIVD and in patients with PSP. Prospective functional studies are needed to ascertain the clinical relevance of midbrain atrophy in SIVD.