An aneurysmal bone cyst is a tumor-simulating bone lesion, the etiology of which is still unclear and probably inhomogeneous. This type of lesion is mainly observed during the second decade of life and is rarely diagnosed beyond the age of 30 years. It is characterized by hollow spaces consisting of several compartments filled with blood and partially divided by septs consisting of spindle-cell tissue with ample multinuclear giant cells and frequent reactive new bone formation. The locations of preference are the metaphyses of the long bones, the spine and flat bones; however, they can appear in every sort of bone. The radiological picture is characterized by osteolytic expansion of the bone with more or less distinct formation of trabeculae. In the CT image we can often find fluid formation; in MRT images alteration of signals can also be seen, indicating former or fresh bleeding. The primary aneurysmal bone cyst should be distinguished from its secondary counterpart; the latter one is occasionally found in giant cell tumors, chondroblastomas, chondromyxoid-fibromas, osteoblastomas, but also in osteosarcomas. When planning biopsy or therapy, this possibility should be also taken into account. The therapy of choice consists of accurate curettage and autologous or allogenous bone transplantation. To avoid recurrences, curettage should be followed by adjuvant therapy with phenol or cryotherapy. When there are extensive recurrences that can no longer be treated by the surgical method described above, embolization of the nutrient vessels may be curative.(ABSTRACT TRUNCATED AT 250 WORDS)