To integrate neuroimaging, neuropathologic, and neuropsychological findings, computer-assisted morphometry was applied to magnetic resonance images of the corpus callosum in adolescents with Down and Williams syndromes and in control subjects. Callosa of subjects with Down syndrome were distinctively rounded in form, consistent with Down syndrome brachycephaly. These callosa also showed decreased widths throughout their rostral fifth, which serves frontal lobe projections. This finding correlates with the hypocellularity and hypofrontality of neocortex in subjects with Down syndrome and with their neuropsychological profile of frontal lobe dysfunction. Callosa of subjects with Williams syndrome generally resembled control specimens, in congruence with their frontal lobe structure and better preserved frontal lobe function. These results represent a convergence of findings across levels of neuroscientific investigation.