Neuroanatomic characteristics of the brains of individuals with Down9s syndrome (DS) are typically characterized at autopsy. Apparent anatomic differences in brain between DS and normal individuals are observable upon gross inspection of MRIs. Area measurements from coronal MRIs are used in this study to determine quantitative structural differences that may occur in DS and during aging. In DS individuals, a significantly larger parahippocampal gyrus, and smaller hippocampus and neocortex, are reported relative to age-matched controls. We also examine two DS individuals with a clinical diagnosis of dementia who exhibit atrophy similar to that typically observed in Alzheimer9s disease. MRI appears to be a useful tool for quantitative anatomic analysis and may be beneficial for determining baseline anatomic measures that can be useful in the diagnosis of changes associated with dementia.