413 patients with symptomatic knee osteoarthritis and low vitamin D levels were randomized to either daily vitamin D3 supplementation for 24 months or placebo. The purpose of this study was to compare clinical outcomes and MRI results on tibial cartilage volume between groups. Results were assessed after 24 months. Primary outcomes of Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) Pain scores and tibial cartilage volume did not significantly differ between groups. Secondary clinical/functional outcome measures and effusion-synovitis volume favoured the vitamin D3 group compared to the placebo group.