Quantitative studies of brain morphology in a group of subjects with Williams syndrome revealed a distinctive pattern of dysmorphology unlike that observed in another form of mental retardation, Down syndrome. Reduced cerebral size but normal cerebellar size was observed in Williams syndrome, in contrast to reductions in both brain components in Down syndrome. Examination of cerebellar vermal morphology suggested significantly increased area of neocerebellar vermal lobules in Williams syndrome, with low-normal size in the paleocerebellar vermal lobules. Thus, a highly selective effect on brain development appears to accompany Williams syndrome, with some brain subsystems, possibly later-developing ones, relatively spared. © 1990, American Medical Association. All rights reserved.