The aneurysmal bone cyst is the result of a specific pathophysiologic change, which is probably the result of trauma or a tumor-induced anomalous vascular process. In approximately one third of cases, the preexisting lesion can be clearly identified. The most common of these is the giant cell tumor, which accounts for 19-39% of cases in which the preceding lesion is found. Other common precursor lesions include osteoblastoma, angioma, and chondroblastoma. Less common lesions include fibrous dysplasia, fibroxanthoma (nonossifying fibroma), chondromyxoid fibroma, solitary bone cyst, fibrous histiocytoma, eosinophilic granuloma, and even osteosarcoma. Interestingly, some of the controversy surrounding this lesion may be the result of a change in how the lesion was defined by Lichtenstein in 1953, when intramedullary lesions were added to the previously described juxtacortical (superficial) lesions. Members of the AFIP have suggested that many of the intramedullary lesions in which no previous lesion can be identified may represent giant cell tumors of bone. Their similarity to proved giant cell tumors in skeletally immature patients can be striking and seems more than coincidental. Appropriate treatment of an aneurysmal bone cyst requires the realization that it results from a specific pathophysiologic process, and identification of the preexisting lesion, if possible, is essential. Clearly an osteosarcoma with superimposed secondary aneurysmal bone cyst change must be treated as an osteosarcoma, and giant cell tumor with secondary features of aneurysmal bone cyst would be expected to be more likely to recur locally. The vast majority (approximately 80%) of patients presenting with aneurysmal bone cystlike findings are less than 20 years old. More than half of all such lesions occur in long bones, with approximately 12-30% of cases occurring in the spine. The pelvis accounts for about half of all flat bone lesions. Most patients present with pain and/or swelling, with symptoms usually present for less than 6 months. The imaging appearance of aneurysmal bone cyst reflects the underlying pathophysiologic change. Radiographs show an eccentric, lytic lesion with an expanded, remodeled "blown-out" or "ballooned" bony contour of the host bone, frequently with a delicate trabeculated appearance. Radiographs may rarely show flocculent densities within the lesion, which may mimic chondroid matrix. CT scanning will define the lesion and is especially valuable for those lesions located in areas in which the bony anatomy is complex, and which are not adequately evaluated by plain films. Fluid-fluid levels are common and may be seen on CT scans and MR images.(ABSTRACT TRUNCATED AT 250 WORDS)