Objectives: To assess long-term outcome and predictors of prognosis following total knee arthroplasty (TKA) for osteoarthritis. Methods: We followed-up 325 patients from 3 English health districts approximately 6 years after TKA, along with 363 controls selected from the general population. Baseline data, collected by interview and examination, included age, sex, comorbidity, body mass index (BMI), functional status and preoperative radiographic severity of osteoarthritis. Functional status at follow-up was assessed by postal questionnaire. Predictors of change in physical function were analysed by linear regression. Results: Between baseline and follow-up, patients reported an improvement of 6 points in median Short Form 36 Health Survey (SF-36) physical function score, whereas in controls there was a deterioration of 14 points (pConclusions: Improvements in physical function following TKA for osteoarthritis are sustained beyond 5 years. The benefits are apparent in patients who are obese as well as non-obese, and there seems no justification for withholding TKA from obese patients solely on the grounds of their body mass index.