The article presents the clinical case of a IIIrd stage giant cell tumor (malignant clastroma) with diaphyseal localization. In almost all published studies, the giant cell tumor has only epiphyseal localization with metaphysis invasion. In this case, the diaphyseal localization created several complications in the establishment of a correct diagnosis and treatment. The patient, a relatively young person without oncological history, was diagnosed with a femural shaft fracture and was correctly treated with an intramedullar nail, followed by biopsy and a radical surgical intervention. The results of the anatomopathological and cytological examinations offered a major surprise, evidencing a giant cell tumor in aggressive form.