In a retrospective study of forty-nine rheumatoid patients followed for five to seventeen years after synovectomy in sixty-six knees, the effects of the operation and the influence of progression of the disease on the knees operated on were assessed as well as the final results, which were good in twenty-six, satisfactory in eighteen, unsatisfactory in twenty-two, and poor in nine knees. The findings indicated that: (1) results are better when involvement of the knee is not advanced at the time of surgery; (2) progressive systemic deterioration after operation does not preclude a satisfactory result; (3) the results at two years after synovectomy deteriorate mildly thereafter in a few patients only; (4) a flexion deformity of more than 15 degrees portends a poor prognosis; (5) age at operation is not a significant consideration; and (6) synovectomy may have a prophylactic effect.