Summary:Eight children developed osteochondroma (OS) at a mean of 88 months after hematopoietic stem cell transplantation (HSCT). The mean age at HSCT was 56 months (12-84). This represents a cumulative incidence of 20% among patients less than 18 years of age transplanted from 1981 to 1997. These eight patients underwent allogeneic (n = 2) or autologous (n = 6) transplantation for either acute leukemia (n = 6) or neuroblastoma (n = 2) after a conditioning regimen including TBI (n = 7) or a combination of Bu and CY. OS was multiple in seven patients and solitary in one. Eight lesions were resected and all were benign. Four children received growth hormone before diagnosis of OS, but there was no clinical, radiological or histological difference between those who did not. Univariate analysis showed an increased rate associated only with autologous HSCT, with a 31.7% probability of a new OS at 12 years after HSCT. Osteochondroma should be added to the other adverse effects of HSCT in children.