A clinicopathological review of 105 cases of aneurysmal bone cyst is reported. Age and sex distribution and localization were in agreement with two previously reported large series. Recurrence occurred in 30.5% of all the cases and in 34.2% of those treated by curettage. Proved incompleteness of surgical removal was an important factor in recurrence (12 out of 17 cases or 70.6%). The recurrence rate was significantly (P2 < 0.01) higher in cases with a mitotic index of seven or more per 50 fields (X 750) than in those with a lower mitotic index. In relation to the recurrence rate, no statistically significant differences were found between the age groups < 15 and > 15 years, between the combination age and lesion size, or between four localization groups. The cyst wall and the septa consist of a superficial layer of cellular fibroblastic and histiocytic tissue in which multinuclear giant cells are present, covering a deeper situated zone of less cellular fibrous tissue. This typical layered architecture was found at one or more places in 93 (88.6%) out of 105 cases. However, layered architecture may also be found in cases of teleangiectatic osteosarcoma. It is suggested that the group with a high risk of recurrence be treated more radically, i.e., with cryosurgery.