This study evaluated the influence of intra-operative soft tissue balance on post-operative range of motion in patients undergoing posterior-stabilized total knee arthroplasty with navigation. After implantation of all components in 31 consecutive patients, the joint component gap was measured manually with the knee at 0 , 90 , and 120 as guided by a navigation system. We designated soft tissue balance as the absolute difference between varus stress and valgus stress (medio-lateral laxity). Changes in medio-lateral laxity were calculated from 120 to 90 , 120 to 0 , and 90 to 0 . Correlations among post-operative flexion angles and pre-operative flexion angles, intra-operative flexion angle after implantation, soft tissue balance, and the changes were analyzed. The mean preand post-operative knee flexion angles were 114 ± 20 and 127 ± 9 , respectively. The mean intraoperative flexion angle was 137 ± 6. The post-operative flexion angle was positively correlated with the pre-operative flexion angle and intra-operative flexion angle after implantation. The change in soft tissue balance between 120 and 90 showed a positive correlation with the postoperative flexion angle. In conclusion, Assessment of intra-operative soft tissue balance could predict post-operative flexion angle. ARTICLE HISTORY