Data of 64 consecutive patients with malignant tumors of the knee treated with tumor endoprostheses between 1976 and 1996 were reviewed. The study was retrospective before 1990 and prospective since then. Four patients were lost to followup and are not included in the study. The average age of the remaining 60 patients was 33.2 years (range, 9-72 years). Fifty patients had surgery for primary sarcoma and 10 patients had surgery for metastases of carcinoma. In the patients in the sarcoma group, the probability of 5 years survival was 72% and 10 years survival was 69%. The overall clinical score of 47 patients with a followup more than 9 months was 81% using the evaluation system of the International Society of Limb Salvage. Fifty-eight reoperations were done in 29 patients because of complications. Twenty reoperations were attributable to soft tissue complications, 34 reoperations were because of mechanical problems related to the prostheses, and four were because of local tumor recurrence. The probability of survival of the leg of the patient after 10 years was 95%. Despite a high rate of complications after limb salvage with endoprostheses, the final clinical outcome usually was good.