A group of 639 bone lesions was reviewed in order to study the features of the aneurysmal bone cyst and its association with other conditions. A diagnosis of primary aneurysmal bone cyst not associated with any other bone lesion was made in 87 patients. In 36 additional patients the gross and microscopic changes of aneurysmal bone cyst were identified as part of some other solid bone lesion. Fourteen of these additional cases were associated with giant cell tumor (96 cases studied), six with chondroblastoma (41 cases studied), three with chondromyxoid fibroma (45 cases studied), two with nonossifying fibroma (68 cases studied), four with osteoblastoma (61 cases studied), one with fibrosarcoma (50 cases studied), three with fibrous histiocytoma (45 cases studied), two with osteosarcoma (100 cases studied), and one with fibrous dysplasia (42 cases studied). The age, site, and sex distribution of the cases associated with another lesion compares closely with that of the solid lesion concerned, supporting the concept that the aneurysmal bone cyst component is secondary.