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Giant cell tumor (GCT) of bone was first described almost 200 years ago, but the optimal treatment continues to evolve. We present a patient with a pelvic GCT who was treated with embolization, 20 months of denosumab therapy, and resection. Histologically, the tumor consisted of degenerated GCT, bone, and fibrous tissue. After 7 months, the patient was found to have osteosarcoma at the site of the initial lesion as well as pulmonary metastases.The apparent malignant transformation of a GCT of bone treated initially with denosumab indicates that close follow-up is warranted.