[A rapidly expanding epithelial skull cyst of uncertain origin]

A rapidly expanding huge cystic lesion of the skull was the cause of unilateral exophthalmus in a 33 year old man. The differential diagnosis included an aneurysmal bone cyst, keratinous cyst, leptomeningeal cyst and intraosseous hemangioma. Although an epithelial lining was verified, the exact nature of the cyst could not be clarified despite immunohistochemical and electron microscopical investigations.