A 29-year-old male was referred for assessment of an expanding left parietal skull lesion. Physical examination revealed a tender, cystic lesion in the left parietal area and a firm, nontender protuberance of the left parieto-occipital skull. Skull X-rays showed a zone of radiolucent defects and extensive areas of bony sclerosis. Computed tomography disclosed an area of bone lysis and areas of bony sclerosis. A bone scan with ^99mtechnetium polyphosphate demonstrated a defect and extensive uptake. External carotid and vertebral angiograms showed dilatation of the meningeal artery, faint accumulation of contrast medium, aneurysmal distension, and early-filling veins in the areas of bony sclerosis. At surgery, a hemorrhagic, cystic lesion, which had completely eroded the inner and outer tables of bone, was found. It was surrounded by thickened, soft, gritty skull bone. Micro-scopic studies revealed cavernous spaces surrounded by multiple benign-appearing giant cells characteristic of an aneurysmal bone cyst, as well as regions of fibrous dysplasia. The difficulties in the differential diagnosis of cyst formation in fibrous dysplasia and aneurysmal bone cyst are discussed.