Mycophenolate Mofetil (MMF), the morpholinoethyl ester of the active compound mycophenolic acid, is a new immunosuppressive agent which blocks the de novo synthesis of guanine nucleotides. As this pathway is essential for purine synthesis of both T and B lymphocytes, the blocking effect on cell division is lymphocyte-selective with little effect on the division of other cell types. In three recent randomized multicenter trials, MMF in conjunction with ciclosporin and steroids has been shown to reduce the incidence of early acute rejection after renal transplantation by almost 50%. The toxicity profile is comparable to that of azathioprine but with a significantly higher efficacy. Major side effects are gastrointestinal with a slight increase of the incidence of mainly viral (CMV) infections. In contrast to ciclosporin and FK-506, MMF is not nephrotoxic. Future studies have to show whether the use of MMF in combination with other immunosuppressants may allow to reduce the doses of the most toxic ones. In addition, long-term studies are needed to prove its potential protective effect on the occurrence of chronic rejection as suggested by several experimental studies and by its effect on the incidence of acute rejection.