Introduction. The main indication for knee arthrodesis in tumor surgery is a tumor that requires an extensive resection in which the joint surface cannot be preserved. We report a patient that had knee desarthrodesis 41 years after giant cell tumor resection followed by a knee arthrodesis. This is the longest reported follow-up after desarthrodesis and conversion to total knee arthroplasty (TKA), almost ten years. Case Report. A 71-year-old man with a distal femoral giant cell tumor had undergone a resection of the distal femur and knee arthrodesis using Kuntscher nail in 1962. In July 2003 he experienced gradually increasing pain of his left knee. We performed a desarthrodesis and conversion to TKA in 2005. The postoperative period passed uneventfully as his pain and gait improved, with gradually increasing range of motion (ROM) and no infection. He now walks independently, with no brace or contractures. Conclusion. Desarthrodesis of the knee joint and conversion to TKA are a difficult surgical choice with a high complication risk. However, our patient's life style has improved, he has no pain, and he can ascend and descend stairs more easily. The surgeon has to be very meticulous in selecting a patient for knee arthrodesis and counseling them to realize that their expectations may not be achievable.