The article presents the case of a 41-year-old patient with a tumor of the distal third of left femur. Following extended radiological diagnostic work-up, MR, and an open biopsy, the diagnosis of a giant cell tumor was made. In view of the patient's clinical status, diagnostic work-up, and considerable tumor size, the decision was made to resect the tumour and use a primary custom-made endoprosthesis. A good clinical outcome was achieved. Even with the extended diagnostic work-up, the determination of tumor malignancy still posed a diagnostic problem.