Background Tai chi is a form of complementary and alternative medicine with similarities to aerobic exercises, which has been recommended for relieving cancer-related symptoms. The aim of this systematic review is to summarize and critically evaluate the evidence available from controlled clinical trials of tai chi as a supportive therapy for cancer patients. Methods We have searched the literature using 19 databases from their respective inceptions through October 2006, without language restrictions. Methodological quality was assessed using Jadad score. Results The searches identified 27 potentially relevant studies. Three randomised clinical trials (RCTs) and one non-randomised controlled trial (CCT) met our inclusion criteria. All of these trials assessed patients with breast cancer. Two RCTs reported significant differences in psychological and physiological symptoms compared to psychosocial support control. Most trials suffered from methodological flaws such as small sample size, inadequate study design and poor reporting. Conclusion The evidence is not convincing enough to suggest that tai chi is an effective supportive treatment for cancer. Further research should attempt to answer the many open questions related to the usefulness of tai chi for supportive cancer care.