Allogeneic hematopoeitic progenitor cell transplantation (HPCT) is a curative therapy for pediatric patients with both malignant and nonmalignant diseases. Single or multiple benign exostoses or osteochondromas have been reported after total body irradiation (TBI), as well as after focal irradiation. Patients exposed to TBI at a young age are at highest risk of developing exostoses. The objective of this institutional review board-approved study was to look at potential factors, besides radiation, that may play a role in development of exostoses. All patients who underwent allogeneic and autologous HPCT at a single institution between March 1992 and December 2003 and who developed an exostosis identified by clinical findings or as an incidental finding on a radiologic study were included. A case-control design matched patients with controls who had the same stem cell source.