The size of the corpus callosum was assessed visually and by computer-assisted image analysis in a series of 445 consecutive magnetic resonance imaging (MRI) scans in children under 17 years of age. Fifty individuals were subjectively identified with small corpora callosa on visual inspection of the MRI scans. Seven patients had true hypoplasia of the corpus callosum after comparing the computer-measured relative size of the structure to previously established normal values. Five additional patients had complete agenesis, and two had partial agenesis of the corpus callosum. The cognitive functional levels of the seven patients with callosal hypoplasia and a control group of 63 randomly selected individuals from the remainder of the group were ascertained by record review. Seventy-one percent of the patients with hypoplasia of the corpus callosum and 29% of the control group had impaired function. The P value determined by Fisher's exact test was P = .061, suggesting that further study with greater numbers may be warranted. The prevalence of mental retardation in this condition, and the fact that hypoplasia is as common as complete and partial agenesis of the corpus callosum combined, suggest that hypoplasia of the corpus callosum is a marker of cerebral dysgenesis that should be looked for in the appropriate clinical setting.