We investigated the effects of tai chi on disease activity, flexibility and depression in patients with ankylosing spondylitis (AS). We allocated 40 patients to either a tai chi treatment group or a no-treatment control group. The tai chi group performed 60 min of tai chi twice weekly for eight consecutive weeks and 8 weeks of home-based tai chi, after which the group showed significant improvement in disease activity and flexibility compared to the control group. All outcome measures were significantly lower in the tai chi group than they were during pre-treatment, while they did not change in the control group. These findings suggest that tai chi can improve disease activity and flexibility for patients with AS. Tai chi is an easily accessible therapy for patients and, as such, may be an effective intervention for AS. However, we cannot completely discount the possibility that the placebo effect was responsible for the improvement.