Mycophenolate mofetil (formerly known as RS-61443) is a morpholinoethyl ester of mycophenolic acid. Mycophenolic acid is a unique immunosuppressive agent because of its mechanism of action. By inhibiting the de novo pathway of purine synthesis, mycophenolic acid suppresses lymphocyte function much more than that of neutrophil, erythrocyte, and other rapidly dividing cell lines which can use the salvage purine synthesis pathway. Mycophenolate mofetil has been shown to be an effective immunosuppressive agent in both animal models and early human trials. This article describes the history, biochemistry, pharmacology, in vitro and in vivo effects, and clinical trials of myophenolate mofetil. In addition, previously unpublished 3-year follow-up of heart transplant recipients receiving mycophenolate mofetil is reported.