Three cases of bone sarcomas arising in fibrous dysplasia (FD) without prior radiation therapy were reported. One of the tumors was a conventional osteoblastic osteosarcoma (OS) and the other two were malignant fibrous histiocytomas (MFH). The lesion of FD was monostotic in two patients and polyostocic (monomelic) in one. Bone sarcomas and FD were detected simultaneously in all cases. One of the MFH cases died 6 months after diagnosis, and the other MFH case and the OS case are alive without disease 3 years and one and a half years after surgery, respectively. Conservative treatment has been selected in most of the patients of FD. We should closely follow up these patients, because bone sarcomas such as OS and MFH could arise at the site of FD. This is the first report of the cases of MFH of bone arising in FD.