A massive prosthesis and medial gastrocnemius muscle transfer were used to reconstruct the knee after extracapsular en bloc excision for bone sarcoma. Magnetic resonance images showed intraarticular involvement. This technique was used in nine patients, six men and three women aged 18 to 51 years, with primary malignant bone tumors of the knee. Extraarticular resection of the knee, including the patella, was done in every case. A knee prosthesis was implanted, and the extensor mechanism was reconstructed by transfer of the medial gastrocnemius muscle and pes anserinus tendons. All resections had negative margins. There were no local recurrences, but metastases occurred in two patients. Infection was the only major complication and was seen in two patients. The mean postoperative Musculoskeletal Tumor Society score was 61% (range, 36%-100%). The mean postoperative range of flexion was 62 degrees (range, 30 degrees-90 degrees), and the mean extensor lag was 12 degrees (range, 0 degrees-40 degrees). Three patients required a crutch to walk. The functional outcome was poor in the two patients whose proximal tibia was removed with the joint, suggesting that arthrodesis may be best in this situation. In properly selected patients, prosthesis and muscle flap reconstruction provides acceptable function and a good cosmetic result.