This retrospective study investigated the modes of implant failure in 80 patients with modular oncology knee prostheses. Twenty patients (25%) required revision: 12 (60%) for stem loosening, 6 (30%) for bearing failure, and 2 (10%) for infection. Patients with bone sarcomas survived longer; however, long-term prosthetic survivorship was a problem. A higher failure rate was found in patients with tibial tumors and with adjuvant treatment of chemotherapy. This study demonstrates for improved long-term survivorship of modular oncology knee prostheses, there must be improvement in the methods of stem fixation, prosthetic materials, and bearing mechanics.