We investigated the relationship of knee range of motion (ROM) and function in a prospective, observational study of primary total knee arthroplasty (TKA). Preoperative and 12-month data were collected on 684 patients, including knee ROM, Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) pain and function questionnaire scores, patient satisfaction, and perceived improvement in quality of life (QOL). Only modest correlations were found between knee ROM and WOMAC function (r<0.34). At 12 months we found significantly worse WOMAC function scores for patients with <95° flexion compared with patients with ≥95° (mean, 61.9 vs 75.0; P<.0001). In linear regression models, WOMAC pain and function scores at 12 months were both correlates of patient satisfaction and perceived improvement in QOL (standardized beta>3.5; P<.0001), but knee flexion was not. For assessment of these outcomes, WOMAC function appears to be more important than knee flexion.