In a comprehensive review of the records of more than 2,000 primary bone lesions encountered at operation at the Mayo Clinic in the period 1905–52, 26 aneurysmal bone cysts were found. The lesion first was recognized by Jaffe and Lichtenstein (1), this being one of the many contributions they have made in the field of bone pathology. Our investigation reaffirms their belief that aneurysmal bone cyst is a distinct clinical and pathologic entity. The specific term “aneurysmal bone cyst” does not appear in the literature until 1942, when Jaffe and Lichtenstein stated for the first time that it was probably a distinct entity and described the findings in 2 cases. In many papers prior to this (2–7), especially those concerning giant-cell tumors and their “variants,” more or less complete descriptions of the lesion are encountered under a variety of names. Ewing (8), in 1940, employed the term “aneurysmal giant-cell tumor” for the condition, which he considered to be a benign variant of a giant-cell tumor takin...