The purpose of this study was to compare the safety and effectiveness of Tai Chi with an Attention Control intervention consisting of a stretching and wellness education program involving 40 patients with osteoarthritis (OA) of the knee. We hypothesized that the participants receiving Tai Chi would show greater improvement in knee pain, physical and psychological functioning, and health-related quality of life than participants in the Attention Control group, and that the benefit would be mediated by effects on muscle function, musculoskeletal flexibility and mental health.