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 less than 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 less than or equal to 15 and greater than 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.