From the history of the comparatively long course of the illness in this case and from the roentgenographic findings, we may assume that the lesion was originally a benign giant-cell tumor. It is difficult even to speculate when malignancy set in. Although there is a history of injury subsequent to which the patient became acutely aware of the tibial lesion, yet for months previously he had frequently felt a twinge of pain in the leg. As this patient indulged in many athletic sports, perhaps he had injured the leg on many occasions and in that manner had stimulated the malignant change. It may be permissible to believe that the lesion may have had a low grade of malignancy originally, which was further attenuated but not entirely destroyed by the intensive radiation, and that subsequent minor traumata excited a full development of malignancy. Appreciating that many giant-cell tumors of bone have been cured by radiation, nevertheless the author cannot but feel that a more satisfactory procedure in this case, which was readily accessible to surgery, would have been thorough removal of the tumor tissue, supplemented by cauterization of the wound, as advised by the surgeon originally consulted six years ago. Knowing as we do now, through the abundant evidence recorded in the literature, that a giant-cell tumor of bone may become malignant and prove fatal, the author is convinced that when a giant-cell tumor can be treated surgically it should be completely excised at the earliest opportunity. Furthermore, in order that there shall be no doubt about the diagnosis, which cannot be certain from roentgenographic evidence alone, a biopsy ought to be performed in every accessible lesion which may be a giant-cell tumor.