This study examined the role that flexion contracture plays in postoperative outcomes after total knee arthroplasty using a retrospective database review. The relationships between preoperative and postoperative knee extension, walking ability, stair climbing ability, Knee Society scores, pain scores, and knee function scores were studied in 5,622 knees. A preoperative flexion contracture was associated with an increased incidence of a persistent postoperative flexion deformity. A postoperative flexion contracture was associated with poorer postoperative results. Furthermore, a postoperative hyperextension deformity of greater than 10 degrees was associated with an increased risk of suboptimal pain and Knee Society scores. Knee extension deformities play a substantial detrimental role in the functional outcome of primary total knee arthroplasty.