Giant cell tumor of bone is an uncommon benign tumor that frequently recurs locally. Spontaneous 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 41 years after initial curettage and subsequent resection. A 68-year-old man presented with a 6-month history of left hip pain. He had been diagnosed 41 years previously with giant cell tumor in the left femoral neck treated by simple curettage and bone grafting, followed by resection of the femoral head 1 year later for local recurrence. On presentation, radiographs revealed a destructive lesion in the left proximal femur. Incisional biopsy revealed recurrence of giant cell tumor with suspected malignant transformation. The patient underwent en bloc resection of the proximal femur with adequately wide margins and reconstruction of the hip joint with a prosthesis. Pathological findings showed malignant transformation of a giant cell tumor to osteosarcoma and leiomyosarcoma. No recurrence or metastasis developed during 2-year follow-up. Benign local recurrences usually arise in the first 3 postoperative years, whereas malignant transformation tends to take longer than 3 years. To the authors' knowledge, the 41-year interval from primary surgery to diagnosis of malignancy for the current patient is the longest interval reported among cases in which patients received no radiation therapy.