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Abstract Objective  The objective of this study was to use magnetic resonance imaging to develop an improved morphological understanding of the abnormalities of the forebrain in Chiari II malformation. Materials and methods  Seventy-four patients with Chiari II malformations investigated between 1999 and 2007 were enrolled. Imaging was retrospectively reviewed by two pediatric neuroradiologists, with special attention given to diencephalon, midline commissures, hemispheric white matter, and cortex. Results  An abnormal gray matter structure that we called hypothalamic adhesion across the anterior-inferior portion of the third ventricle was noted in 48.6%. The anterior commissure was in a low position in the lamina terminalis in 38%. Gross abnormalities of the corpus callosum/hippocampal commissure were found in 57%; they were, however, different from the abnormalities seen in classical commissural agenesis. An abnormal bundle of white matter forming a callosal ridge was noted on the dorsal callosal surface in 60%; it is believed to represent the aberrant cingular bundle recently identified using diffusion tensor imaging. Hemispheric white matter could be considered as normal in 48%, deficient posteriorly in 55%, and globally in 10%. Cortical posterior medial stenogyria was observed in 72%. Gray matter heterotopias were found in 19%. The posterior limbic cortex was thin and dysplastic in 46%. Hippocampi were commonly abnormal (85%), with atypical sulcation of the adjacent temporo-mesial cortex (93%). Conclusion  Major structural abnormalities were common in Chiari II malformation and were probably not related to hydrocephalus. Important anatomical structures involved in neurocognitive function should be considered as factors in the prognostic assessment of Chiari II patients. [ABSTRACT FROM AUTHOR]