1. Synovectomy was carried out in thirty-four knees, of which thirty-one were certainly or probably afflicted by rheumatoid arthritis. 2. Synovectomy was considered only when adequate medical and physical treatment had failed to afford relief. Its purpose is to preserve useful function, and one of the principal factors influencing the decision to operate was the retention of a normal or good radiographic joint space in a persistently painful, warm and swollen knee. 3. Radiographic appearances constituted a useful but not infallible guide to the true state of the articular surfaces. 4. The method of operation, findings and subsequent care are described. 5. Up to two years after operation it appeared that improvement might be related to the state of the articular surfaces, but after two years this relationship was not evident and an attempt to explain this difference has been made. 6. It seems clear that the longer-term results are determined mainly by the course taken by the rheumatoid process. An unfavourable course was associated with considerably less satisfactory results than was a favourable course. 7. It is concluded that in rheumatoid arthritis which has resisted rest and medical treatment synovectomy of the knee is most likely to be successful when the radiographic joint space is good or normal, and when the rheumatoid process follows a favourable course. To undertake the operation at an early stage in the disease is to leave in doubt the outcome, as this is dependent upon the as yet undeclared general course of the disease. Even so,
