Osteoarthritis (OA) of the knee is the most frequent cause of knee pain after the age of 50 years. OA is a joint disease characterised by articular cartilage loss associated with structural changes in the cartilage and adjacent structures. The main symptoms are pain and functional disability. The goals of OA therapy are to decrease pain and maintain or improve joint function. There is evidence that diacerein has both a symptomatic and a structural effect on cartilage, and clinical studies suggest that diacerein therapy significantly decreases OA symptoms when compared to placebo. Diacerein has been shown to inhibit interleukine-1 (IL-1β), and down-regulated IL-1β stimulated secretion of metalloproteinases and aggrecanases, and thereby prevent breakdown of cartilage by these enzymes. Diacerein has no effect on the synthesis of prostaglandins, and therefore no effect on the upper intestinal tract. The purpose of this phase III-IV international, multicentre, double-blind, non-inferiority, randomised, controlled study is to determine the efficacy and safety of diacerein vs. celecoxib on symptoms after 6 months of treatment, and on structural changes after 2 years of treatment in knee OA patients as assessed by magnetic resonance imaging (MRI).