Most bone sarcomas arise in apparently normal bone. However, some sarcomas arise in preexisting benign bone tumors or in nonneoplastic conditions. Some of the lesions, such as multiple exostoses and Ollier's disease, have a propensity to develop chondrosarcoma. Rarely does a benign giant cell tumor become malignant without prior irradiation; malignant transformation of other benign tumors, such as osteoblastoma and chondroblastoma. is a medical curiosity. Among nonneoplastic conditions, radiation changes, long-standing chronic osteomyelitis, and Paget's disease have definite premalignant connotations. However, the risk of developing cancer in these conditions is still low.