This tumor, found most often in children or young adults, consists of a cystic lesion with large vascular spaces,characterized by aggressive, expansile lysis of bone. The tumor is composed of fibrous tissue, vascular spaces with alining resembling endothelium, giant cells, and reactive bone formation at the periphery. Aneurysmal cysts can arise as asecondary degenerative vascular lesion within another primary benign or malignant bone tumor, such as giant cell tumor ochondroblastoma; however, about half are thought to represent primary lesions. Because recurrence is relatively frequentwith simple curettage, local resection with bone grafting is preferable. Embolization has been used successfully inunresectable spinal or pelvic lesions, as has intermediate-dose radiation treatment. Preoperative embolization of largelesions is helpful in decreasing the risk of hemorrhage.