A rare case of idiopathic massive osteolysis of the skull is reported. A 49 year-old male was admitted with a diagnosis of a skull tumor. The skull film taken for evaluation of the headache, showed a large lytic lesion with irregular margin in the left parietal area. CT showed that some of the bone had been destroyed, but the underlying brain was normal. Technetium 99m-MDP bone scan showed high activity around the margin of the lytic lesion. During surgery for this bone lesion no neoplastic or inflammatory changes were revealed. However, destruction of the bone without new bone formation could be seen. Idiopathic massive osteolysis of skull bone is extremely rare and the authors could find only 6 reported cases in the literature. It is characterized by the spontaneous onset of bone resorption without known causative factors. This destructive process continues for years until eventually it ceases spontaneously. In the case of skull lesion, there seems to be no need for surgery unless both the inner and outer tables are involved. This disease must be included in the differential diagnosis of lytic skull lesion.