OBJECTIVE: Aging in Down’s syndrome is accompanied by amyloid and neurofibrillary pathology, the regional and laminar distribution of which resembles pathological changes seen in Alzheimer’s disease. Previous studies using magnetic resonance imaging (MRI) demonstrated age-related atrophy of medial temporal lobe structures in nondemented older subjects with Down’s syndrome, reflecting early allocortical pathology. Corpus callosum atrophy has been established as a marker of neocortical neuronal loss in Alzheimer’s disease. This study investigated whether atrophy of the corpus callosum and hippocampus occurs in nondemented subjects with Down’s syndrome and compared the degree of age-related atrophy between these structures. METHOD: Hippocampus and corpus callosum measures were obtained from volumetric T1-weighted MRI scans of 34 nondemented Down’s syndrome adults (mean age=41.6 years, 17 women) and 31 healthy comparison subjects (mean age=41.8 years, 14 women). RESULTS: Down’s syndrome subjects had smaller c...