Fibrous dysplasia (FD) is a relatively uncommon disorder that affects primarily the cranial region; its occurrence in the cranial base in combination with hindbrain herniation and aneurysmal bone cyst (ABC) constitutes an extremely rare condition. We report a case of polyostotic fibrous dysplasia with progressive occipital, temporal, and clival involvement. Clinical findings and differential diagnosis with special emphasis on the imaging features were discussed. A small posterior fossa volume has been thought to lead to hind brain herniation. The resultant obstruction to the CSF pathways at the level of the foramen magnum has been implicated in the development and subsequent progression of syringobulbia.