Patients with late-onset Alzheimer's disease (AD) have more white matter changes on magnetic resonance imaging (MRI) than controls. To test the hypothesis that AD patients might have also atrophy of the corpus callosum (CC), we compared the CC thickness on MRI from 20 AD patients and 21 controls. We found a significant reduction in the CC thickness in AD compared with age-matched controls (p < 0.01). We demonstrated that atrophy of the CC depends mainly on the diagnosis of senile dementia of the Alzheimer type and at a lower degree on the diagnosis of presenile AD but neither on age nor on ventricle enlargement. This result suggests that beside the greater severity of white matter involvement in late-onset AD, atrophy of the CC may also be present.