We developed the Hy-Flex II total knee and ligament balancing system (Hy-Flex II total knee) which provides (1) a flexion angle of at least 120 degrees or more, (2) a range of motion (ROM) comparable to or greater than that before surgery, and (3) occasional full flexion. The system design has several features: small posterior femoral condyle radius, a 4 degrees posterior tilting of the tibial joint surface, and equal tension of the bilateral soft tissues obtained by using a ligament tensor. From September 1997 to June 1999, Hy-Flex II total knee arthroplasty was carried out on 114 joints of 84 patients with rheumatoid arthritis. One year after surgery, the average flexion in our series was 122.1 degrees +/-15.0 degrees. The number of knees operated on which had a flexion angle of 120 degrees or greater was 82 (71.9%) 1 year after surgery among the total of 114 knees. In total, 94 (82.5%) joints obtained the same level or better than the preoperative flexion; 26 (22.8%) attained full flexion 4 weeks after the operation and 15 (13.1%), 1 year after the operation. These results suggest that this system will be able to achieve our aims in almost all rheumatoid knees.