Introduction. The management of degenerative arthritis of the knee in the younger, active patient presents a challenge to the orthopaedic surgeon. Surgical treatment options include: high tibial osteotomy (HTO), unicompartmental knee arthroplasty (UKA) and total knee arthroplasty (TKA). The aim of this study was to examine the long-term survival of closing wedge HTO in a large series of patients up to 19 years after surgery. Methods. Four hundred and fifty-five consecutive patients underwent lateral closing wedge HTO for medial compartment osteoarthritis (MCOA) between 1990 and 2001. Between 2008-2009, patients were contacted via telephone. Assessment included: incidence of further surgery, current body mass index (BMI), Oxford Knee Score, and British Orthopaedic Association (BOA) Patient Satisfaction Scale. Failure was defined as the need for revision HTO or conversion to UKA or TKA. Survival analysis was completed using the Kaplan-Meier method. Results. High tibial osteotomy survival was determined on 413 patients (91%) and, of the 397 patients who were alive at the time of final review, 394 (99%) were contacted for follow-up via telephone interview. The probability of survival for HTO at 5, 10 and 15 years was: 95%, 79% and 56% respectively. Multivariate regression analysis showed that age < 50 years (p=0.001), BMI < 25 kg/m. 2. (p=0.006) and ACL deficiency (p=0.03) were associated with better odds of survival. Mean Oxford Knee Score was 40/48 (range 17-48). Overall, 85% of patients were enthusiastic or satisfied and 84% would undergo HTO again at mean 12 years follow-up. Conclusion. High tibial osteotomy can be effective for periods longer than 15 years. However, results do deteriorate over time. Age < 50 years, normal BMI and ACL deficiency were independent factors associated with improved long-term survival of HTO