To assess any clinically important difference in functional outcome over 10 years after primary total knee arthroplasty (TKA). A prospective registry-based observational cohort study including 309 patients older than 60 years who underwent primary TKA. Patients were assessed at 1, 3, 5, 7 and 10 postoperative years with the Knee Society scores (KSS) and Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC). Clinically important improvement was defined according to the minimal clinically important difference (MCID). Patients were also categorized as type A (unilateral knee osteoarthritis), type B (bilateral knee osteoarthritis) or type C (various sites of osteoarthritis). The mean age at the TKA surgery was 69.2 (SD 7.3) years, 197 (63.7%) were women.Maximum postoperative improvements in functional scores occurred at 3 postoperative years, remained relatively stable up to 5-year. There were significant decreases in all KSS and WOMAC scores at 7-year follow-up (P = .001), remained stable up to 10-year. At 10-year, functional scores were significantly higher than preoperatively (P = .001). Differences between maximum scores at 3-year and those at 10-year were significantly lesser than MCID in all scores (P = .001). In multivariate analysis, type-C patient at TKA surgery was the only significant predictor of unsuccessful KSS score and dissatisfaction at 10-year follow-up. Primary TKA provides clinically important improvements in functional and quality of life outcomes over 10-year follow-up compared to preoperatively. Although there were statistically significant declines in KSS and WOMAC scores from 3 to 10 years, the differences were lesser than the MCID.