This paper describes a prospective study of replacement of the knee joint with the Walldius endoprosthesis in patients disabled by rheumatoid arthritis. Thirty knees in nineteen patients were so treated and followed for an average of almost three years after operation. Evaluation before and after operation on a subjective, objective, and roentgenographic basis, showed that pain, the most disabling symptom, was eliminated or reduced to non-disabling levels in all patients. The total range of knee motion was not improved, but a more useful arc of motion was obtained. Significant instability did not occur. All patients who were in a bed-chair status preoperatively were able to walk postoperatively. Although settling and loosening of both components of the prosthesis was observed, they did not cause symptoms, and within the time limits of this study did not correlate with the duration of follow-up. A large number of cases, observed under standardized conditions over a long period of time, will be required before a definitive evaluation of this procedure can be made.