Aneurysmal bone cyst is not a new lesion nor is it produced by secondary pathological changes in a primary lesion. Aneurysmal bone cysts can be found in hospital records classified under a wide variety of diagnoses, including bone cysts, giant-cell variants, and atypical giant-cell tumors. It occurs throughout the skeleton but chiefly in the spine and long bones; it tends to occur in the younger-age groups with no definite preference for either sex. It is benign; and, when it occurs in the long bone, it is typically seen in the metaphyseal area, usually eccentrically located (Fig. 7). On the roentgenogram it has an expansile soap-bubble appearance and extends beyond the normal confines of the bone, being outlined by a thin layer of subperiosteal new bone. When it occurs in the spine, it involves chiefly the posterior elements and causes enlargement and distention of their outlines. Grossly, it is an expansile lesion delineated by subperiosteal new bone and filled usually with a pulpy red tissue that bleeds freely. However, it may present as a fluid-filled cyst with relatively scanty tissue but the microscopic characteristics of this tissue will be the same. Microscopically, it consists of a thin shell of subperiosteal bone; vascular spaces, often without endothelial lining, that contain normal-appearing erythrocytes; and, between the spaces, septa composed of active fibrous tissue and various numbers of foreign-body giant cells and osteoid. The walls of the vascular spaces do not contain muscular coats. I believe that the microscopic appearance of the septa is the most characteristic finding. Aneurysmal bone cysts are benign and amenable to treatment by curettage and roentgen therapy. The recurrence rate is low. However, Case 6 must be regarded as a failure of therapy and a recurrence. Subsequent surgery was successful. This paper has further delineated the entity, aneurysmal bone cyst, by presenting six cases which, although they vary in anatomical location and somewhat in gross appearance (for example, two lesions were cystic and filled with fluid), had the same microscopic anatomy in the septa and the blood spaces. The roentgenographic characteristics are demonstrated. The successful results of treatment have been discussed, and the shortest period of follow-up is three years.