Neuroanatomy of fragile X syndrome: the posterior fossa. The occurrence and specificity of posterior fossa abnormalities as measured from magnetic resonance images of the brain were investigated in a group of 14 males with fragile X syndrome and comparison groups consisting of 17 males with other causes of developmental disability and 18 males with normal IQs. The size of the posterior cerebellar vermis was significantly decreased and the fourth ventricle significantly increased in the group of males with fragile X syndrome compared with males in both comparison groups. These neuroanatomical abnormalities appeared to be secondary to hypoplasia rather than atrophy. 