The MR imaging studies of 68 patients who had brain anomalies were reviewed retrospectively to evaluate specific anatomic abnormalities of the corpus callosum. The corpus callosum was abnormal in 32 (47%) of the 68 patients. Excluding patients with the Chiari I malformation, callosal anomalies were present in 30 (68%) of 44 patients. Callosal dysgenesis was most common, followed by callosal atrophy or hypoplasia and complete agenesis. The anterior commissure was present in all patients. On the basis of the known temporal sequence of brain and callosal embryogenesis, we deduced the following regarding the pathogenesis of developmental anomalies: (1) callosal dysgenesis occurs as a result of insults during the formation of its precursors, not during formation of the corpus callosum itself; (2) the Dandy-Walker malformation sometimes occurs as a result of an insult in the eighth week of gestation, several weeks later than has been generally accepted; (3) sphenoidal encephaloceles probably occur as a result of faulty disjunction of neuroectoderm and cutaneous ectoderm at the anterior neuropore; and (4) a complete but atrophic corpus callosum results from an insult to the cortex or white matter after formation of the corpus callosum is complete (18-20 weeks). Callosal anomalies, easily identified on MR, are an important indicator of additional brain anomalies. Analysis of the corpus callosum provides important information about the embryogenesis of brain anomalies and may assist in distinguishing between in utero and perinatal brain insults.