Two cases are reported with osteosarcomatous transformation in fibrous dysplasia, one in the skull and one in the iliac bone. In both cases the patient was known to have longstanding polyostotic fibrous dysplasia; no radiation therapy was ever given. The incidence of sarcomatous transformation in fibrous dysplasia in the files of the Netherlands Committee on Bone Tumours is 0.5%. When strict criteria for spontaneous malignant transformation are used, the actual incidence is probably lower than is suggested.