Ontralateral knee was related to postoperative pain in the index knee. Methods: Seven hundred and seventy-two patients undergoing primary unilateral total knee arthroplasty with use of the Kinemax prosthesis for the treatment of osteoarthritis comprised the cohort (Kinemax Outcomes Study cohort). Patients were assessed preoperatively and were followed for twenty-four months after surgery with use of the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC). We collected separate WOMAC pain scores for the index knee and the contralateral knee. Our primary outcome measure was the WOMAC pain score (rescaled to 100, with 100 being the best score) for the contralateral knee at three, twelve, and twenty-four months. We examined whether within-subject changes in the WOMAC pain score for the contralateral knee were predicted by the WOMAC pain score for the index knee at three months with use of linear regression and multilevel models after adjustment for sex, age, country, body mass index, income, and mental well-being. Results: Improvement in terms of pain was observed in both the index and contralateral knees between baseline and three months. Subsequently, there was a modest deterioration of 3.5 units per year (standard deviation, 9.8 units per year) in the contralateral knee (p < 0.001), which was not predicted by pain in the index knee shortly after surgery (p > 0.6). Conclusions: Pain in the index knee at three months after total knee arthroplasty did not appear to predict a symptomatic increase in pain in the contralateral knee over two years of follow-up in our cohort. The contralateral knee did not require any additional clinical surveillance over and above the patients’ reports on their symptoms. Level of Evidence: Prognostic Level IV. See Instructions for Authors for a complete description of levels of evidence....