Complete gross and microscopic neuropathological examinations of 25 children who died with meningomyelocele, the Arnold-Chiari malformation, and hydrocephalus revealed a wide range and frequency of associated central nervous system malformations. The most remarkable of these anomalies were hypoplasia or aplasia of cranial nerve nuclei (20%), demonstrable obstruction of cerebrospinal fluid flow within the ventricular system (92%), cerebellar dysplasia (72%), a disorder of migration of cortical neurons (92%), fusion of the thalami (16%), agenesis of the corpus callosum (12%), and complete or partial agenesis of the olfactory tract and bulb (8%). The anomalies associated with posterior neural tube closure defects can no longer be considered secondary, but rather must be considered part of a spectrum of malformations caused by an unidentified primary insult to the central nervous system. The frequency and pattern of brain malformations associated with neural tube defects of some children with meningomyelocele suggest that such malformations may seriously affect intellectual outcome.