Osteitis fibrosa cystica occurring as a solitary tumor of the skull is a relative rarity among diseases of the skeletal system. The association of such a lesion localized in the occipital bone with definite visual hallucinatory phenomena and an accompanying hemianopia, together with psychotic manifestations, is considered of sufficient importance to record in some detail. Of particular interest are the facts that the mental symptoms subsided to a noteworthy degree following an operation on the affected area and that the hallucinatory phenomena entirely disappeared. Osteitis fibrosa cystica was called by Locke 1 a rare chronic degenerative disease of bone beginning in late childhood. He stated further that very rarely are cysts found in the skull. Bloodgood 2 specified two distinct types, the generalized or multiple cystic form and the localized or solitary cystic lesion. Their occurrence he believed to be rare before the age of 10 years or after the