Refractory disease occurs in 25% or more of adults with idiopathic (immune) thrombocytopenic purpura (ITP). Therapy to elevate the platelet count may be required in a proportion of these patients. Immunosuppressive agents such as prednisone, azathioprine, cyclophosphamide, and cyclosporin have been shown to be effective treatments in a proportion of patients with refractory ITP. A newer immunosuppressive medication, mycophenolate mofetil (MMF), has been used successfully with acceptable toxicity in solid organ transplant patients to reduce the risk of organ rejection. The goal of this study was to determine whether MMF is an effective treatment for refractory ITP. Efficacy, defined as a sustained platelet increase to a level greater than 50 × 109/L, was seen in 7 of 18 patients with refractory ITP. Three of these 7 patients have had intermittent thrombocytopenic episodes while continuing the medication. No severe toxicity was seen, although two of the 18 patients discontinued MMF within the first month of treatment because of side effects, i.e., headache. In summary, MMF may be a useful component of a combination protocol but does not appear to be highly effective as sole therapy in patients with refractory ITP. The data suggests that response rates to MMF may be higher in patients who have had a shorter duration of their ITP. Am. J. Hematol. 81:19–25, 2006. © 2005 Wiley-Liss, Inc.