This is a 2-year double-blind randomized controlled trial in 287 community-dwelling individuals age 60 years or older undergoing uni-lateral total knee replacement due to severe OA of the knee.We compare an oral dose of 2000 IU vitamin D3 per day to 800 IU. The primary endpoints are pain and disability related to rehabilitation of the operated knee,and related to the expected high prevalence of OA in the contra-lateral knee. Secondary endpoints are change in 25(OH)D levels, muscle strength, balance, lower extremity function, the rate of falls, bone density and bone quality, bone metabolism, general pain, fall-related fractures, body composition, blood-pressure, major cardiovascular events, blood glucose-insulin levels, executive cognitive function, rate of upper respiratory tract infections / all infections, tooth loss, dental health, health care utilization, number and size of facial skin keratosis, and radiographic progression of the non-operated knee. Pain and disability will be assessed by the pain and function subscales of the Western Ontario- McMaster Universities Osteoarthritis Index (WOMAC). Randomization will be stratified by WOMAC function prior to surgery, radiological evidence for OA in the contra-lateral knee, and hospital. Clinical visits will take place at baseline (6-8 weeks after TKR), at 6, 12, 18 and 24 months of treatment. In addition, all individuals will receive a phone call every 2 month to assess adverse outcomes and compliance to treatment, supported by a hotline for immediate report of adverse events.