Three cases of monostotic fibrous dysplasia of the cranial bones are presented. Several theories of the etiology of this uncommon condition are put forward and the relationship between monostotic and polyostotic fibrous dysplasia, Albright's disease, and hyperparathyroidism is examined. The difficulty in diagnosis and the ways they may present to the ear, nose and throat surgeon are discussed. It is suggested that conservative surgery is the best mode of treatment for maxillary or temporal lesions. However, ethmoid lesions, particularly when the optic foramen or base of skull are eroded, may need a neuro-surgical approach. The use of radiotherapy in any case is deprecated.