OBJECTIVES: Total knee replacement (TKR) is a commonly used surgical procedure for patients with severe joint damage caused by arthritis; however, there remain difficulties in establishing criteria to define the optimal timing for TKR. The aim of this study was to identify the preoperative threshold HRQoL scores that were associated with better postoperative outcomes and those associated with greater improvement.  METHODS: Data were collected from 1715 patients undergoing TKR between 2001 and 2006. Patients were interviewed at baseline and at 6 months and 2 years after surgery. At all three interview sessions, patients were asked to complete the Short-Form (SF-36) and the Oxford Knee Score (OKS). As physical function and pain have been identified as the key domains in osteoarthritis, the OKS and the physical functioning (PF) and bodily pain (BP) scales of the SF-36 were selected as the outcome measures of this study. Summary statistics were computed for preoperative, postoperative, and improvement in outcome scores. Visual inspection of the simple error bar charts were used to identify the threshold preoperative scores that were associated with better postoperative scores and those associated with more improvement. RESULTS: Patients with poorer preoperative HRQoL had worse postoperative outcomes but experienced greater improvement after TKR compared to those with better preoperative HRQoL. The highest postoperative PF scores are associated with a preoperative PF score of 50 points and above, whereas the baseline threshold OKS score was between 35 and 40 points. A baseline PF score of < 30 points and an OKS score of > 40 points are associated with the greatest improvement. No clear leveling off pattern was observed in the BP scores.  CONCLUSIONS: PF and OKS threshold scores associated with optimal outcomes of TKR were identified in this study. Future research is needed to examine the predictive value of the scores identified in improving patients’ post-operative outcomes.