Aneurysmal bone cyst is a rare nonneoplastic expansile osteolytic bone lesion of unknown etiology. To the best of the authors' knowledge, no epidemiologic study concerning its incidence has been reported. The authors performed a retrospective, population based analysis of 94 patients with primary aneurysmal bone cyst and a literature review of 1002 patients regarding gender and age predilection. The annual incidence of primary aneurysmal bone cyst was 0.14 per 10(5) individuals. The male to female ratio was 1:1.04, and the median age was 13 years (range, 1-59 years). The results of this study and data compiled from the literature show that aneurysmal bone cysts occur significantly more often in female patients.