Aneurysmal bone cyst (ABC) is a nonneoplastic expansile bone lesion that mainly affects children and young adults. Primary ABC is relatively rare, with an incidence one half that of giant cell tumor of bone. In 238 patients with ABC studied in the Mayo Clinic files, more than 80% of the lesions were in long bones, flat bones, or the spinal column. Of the lesions initially treated at the Mayo Clinic, 95% were typical ABC; the rest were "solid" variants. Except for the absence of obvious cavernous channels and spaces, there was no significant histologic difference between solid variant and typical ABC. Radiographically, ABC is an eccentric expansile lesion commonly located at the metaphysis of long bones. Computed tomography and magnetic resonance imaging may show multiple internal septations or fluid levels. In the 153 patients treated, 19% had recurrence after curettage (intralesional excision). Recurrence was most common during the first 2 postoperative years.