Fibrous dysplasia (FD) is a benign dysplastic pathology of bone-forming mesenchymal cells, resulting in replacement of trabecular bone by abnormal fibrous and immature osseous tissue. FD can be either monostotic or polyostotic, is more often unilateral, and can be part of the McCune-Albright syndrome (MIM 174800). Sarcomatous degeneration in FD is rare, ranging from less than 1% in monostotic forms to 4% in the McCune-Albright syndrome. We report the case of a 59-year-old woman with monostotic FD of the left ischium, known for over 30 years, who developed sarcomatous transformation in a low-grade spindle-cell sarcoma. The value of F-18 fluorodeoxyglucose positron emission tomography in the early diagnosis of malignant transformation, the evaluation of distant metastasis, as well as monitoring the efficacy of chemotherapy are discussed.