Malignant transformation of conventional giant cell tumor of bone is rare and usually occurs with irradiation. This article describes a case of malignant transformation of a giant cell tumor 15 years after initial curettage and bone graft. A 35-year-old man was admitted to the hospital with a recurrent giant cell tumor of the distal femur. On presentation, the patient reported the insidious onset of a dull aching pain in the distal part of the left thigh 4 months prior to admission. Radiographs revealed a destructive lesion in the left distal femur. Needle biopsy revealed recurrence of giant cell tumor with suspected malignant transformation. The patient underwent en bloc resection of the distal femur with adequately wide margins and reconstruction of the knee joint with a prosthesis. Pathological findings showed malignant transformation of a giant cell tumor to high-grade spindle cell sarcoma. Immunohistochemistry showed diffuse and strong p53 expression. A diagnosis of secondary fibrosarcoma was made after discussion. Unfortunately, the tumor proved to be highly resistant to the chemotherapy, and the patient died of multiple lung metastases 14 months after the diagnosis of malignant transformation. What has to be stressed in this case is that any late recurrence must be approached considering the possibility of a secondary induced primary tumor. Because of the rarity of this disease, the effective therapeutic strategy for fibrosarcoma secondary to giant cell tumor is lacking. In addition, identification of the p53 mutation may help in diagnosing cases of potential malignant transformation of giant cell tumor.