Huge arachnoid cyst of the posterior fossa with cerebellar tentorium dysplasia associated with juvenile polyposis.

We report an infant with a huge arachnoid cyst of the posterior fossa with dysplasia of the cerebellar tentorium and meningeal sinus and associated juvenile polyposis. Neuroimaging studies disclosed a huge median cystic lesion extending posterosuperiorly over the cerebellum. The cerebellar tentorium was raised to the parietal area; the vermis was normoplastic.