For over five decades, osteochondromas (exostoses) and associated growth retardation have been known to be caused by radiation damage to the growing skeleton.Patients can be divided into three exposure groups. Group I received external beam radiation therapy primarily for the treatment of childhood cancers (typical dose 3,500 cGy), and 6–20% developed osteochondromas and growth retardation within the radiation portal. Group II consists of recently described patients who received total body irradiation in preparation for bone marrow transplant (typical dose: 800–1,200 cGy), and about 20% developed osteochondromas and growth retardation. Group III consists of 206 German children who in the 1940s and early 1950s received intravenous radioactive Peteosthor (Ra-224) to treat bone tuberculosis (estimated typical dose: 1,000–2,000 cGy), and 14% developed osteochondromas and growth retardation, among other benign and malignant sequelae. Combining these three exposure groups, osteochondromas and growth retardation develop in at least 6–20% of children who receive therapeutic radiation to their growing skeletons.