Aim. To evaluate outcome and complications of knee arthrodesis with a modular prosthetic system (MUTARS(r) Implantcast), as primary and revision implants in musculoskeletal oncology. Method. Between 1975 and 2009, 24 prostheses were used for knee arthrodesis. Nineteen in oncologic cases: 6 osteosarcomas, chondrosarcoma, synovial sarcoma and metastatic carcinoma 3 each, 2 pigmented villonodular synovitis (PVNS), malignant fibrous hystiocitoma and giant cell tumour 1 each. Patients were grouped into: A) primary implants, B) revision implants. Group A included 9 patients: 8 arthrodeses after extra-articular resection with major soft tissue removal, 1 after primary resection following multiple excisions of locally recurrent PVNS. Group B included 15 patients: 12 arthrodeses for infection (5 infected TKAs, 7 infected megaprostheses), 2 for failures of temporary arthodesis with Kuntscher nail and cement, 1 for recurrent chondrosarcoma in previous arthrodesis. Results. Oncologic outcome ata mean follow-up of 6 years (ranging 1 to 26), showed 13 NED (68.4%), 2 NED after treatment of relapse (10.5%), 1 alive with metastases (5.3%), 2 dead with disease (10.5%) and one dead of other disease (5.3%). Complications causing failure were observed in 12 patients (50%): 11 infections at mean of 14 months (6 in arthrodeses as revision for previous infections, 5 in group A), 1 femoral stem breakage at 4.8 years (in group B). Treatment of infections was: amputation in 6, “one stage” in 1, “two stage” with new arthrodesis in 4 (1 subsequently amputated for recurrent infection). The breakage was revised, had further traumatic breakage at 2 years and a second revision. Conclusion. Arthrodesis with modular prosthetic system is indicated after major extra-articular resection or in revisions of severely failed previous reconstructions. High infection rate should be prevented with good soft tissue coverage, by flaps if needed