Twenty seven patients with haemophilia who had repeated haemarthroses affecting the knee joint, despite appropriate substitution therapy, were treated by surgical synovectomy. Open operations were carried out on 18 and 9 had an arthroscopic procedure. The average age at the time of synovectomy was 13 years for both the open and arthroscopic groups. The open group was followed up for an average of 15.2 years and the closed for 5.4 years. Operation took 70 minutes on average in the arthroscopic group and 50 minutes in the open. Patients who had an arthroscopy were in hospital for an average of 5 days and those with an open operation for 10. Synovectomy by both methods significantly reduced bleeding episodes. The results were assessed according to the classification recommended by the Orthopaedic Advisory Committee of the World Federation of Haemophilia. In the open group 2 patients had a good result, 10 were fair and 6 poor; in the arthroscopic group 3 had a good result, 5 were fair and 1 poor. Both operations appeared to slow, but did not halt, the radiographic deterioration of the joints.