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Range of motion is recognized as an important indicator of the success of a total knee replacement; however, an optimal range of motion has yet to be defined. This study was designed to determine the optimal range of motion for knee function after total knee arthroplasty with a posterior cruciate-retaining prosthesis.We retrospectively reviewed 5556 primary total knee arthroplasties performed with posterior cruciate-retaining prostheses between 1983 and 2003. The relationship between postoperative range of motion and pain, walking ability, stair-climbing ability, and knee function scores was examined at three to five years postoperatively. The relationship between a postoperative flexion contracture or hyperextension and knee function was also examined.Patients with 128° to 132° of motion obtained the highest scores for pain, walking, and knee function and the highest Knee Society scores. The outcomes became substantially compromised with motion of <118°. Patients with 133° to 150° of motion had the highest scores for stair-climbing. A postoperative flexion contracture and hyperextension were associated with lower scores for pain, walking, stair-climbing, and knee function.The best functional results following total knee arthroplasty are achieved with 128° to 132° of motion. A postoperative flexion contracture and hyperextension of ≥10° are associated with a poorer outcome except that stair-climbing is improved with more motion.Prognostic Level II. See Instructions to Authors for a complete description of levels of evidence.