A physically and intellectually normally developed boy had a resection of a lumbar myelomeningocele shortly after birth and a shunt was inserted for hydrocephalus. He died at age 12 years after a traffic accident. Autopsy disclosed a hyperplasia of the cerebellar vermis. An abnormal median-sagittal triangular fiber tract ran on the dorsal surface of the corpus callosum and contained more than 3 million fibers, most of them very thin and myelinated. The tract was identified as a hyperplastic fornix dorsalis. The complex malformation was attributed to hyperplasia of midline structures.