A retrospective study was conducted among Italian children treated with hematopoietic stem cell transplant (HSCT) to evaluate the incidence and risk factors in the development of osteochondroma (OC). OC occurred in 27 patients who received autologous or allogeneic HSCT. The estimated 5-, 10-, and 15-year cumulative risk of developing OC was 0.5%, 3.2%, and 6.1%, respectively. Analysis of cumulative risk stratified by the various risk factors revealed that male sex (P=.026), autologous HSCT (P=.001), age at HSCT (< or =3 years) (P < .0001), and total body irradiation (TBI) (P <.0001) significantly affected the risk of OC. Multivariate analysis, restricted only to tumor types with at least 1 case of OC, showed that earlier age at HSCT (P =.0004) and TBI (P < .0001) were the only factors that were significantly associated with OC.