Mycophenolate mofetil (MMF), the morpholinoethyl ester of mycophenolic acid (MPA), is a new selective immunosuppressant used for the prevention and treatment of acute renal rejection after transplantation. In vivo MMF is deesterified to MPA, which is a potent and specific inhibitor of de novo purine synthesis and suppressor of both T and B lymphocyte proliferation. In animal studies, MMF has been shown to be effective in prolonging the survival of allografts and xenografts in rodents, dogs, and monkeys. Experimental evidence in animal models suggests that MMF also may be effective in the treatment of chronic vascular rejection. A phase I clinical trial showed MMF was well tolerated in renal transplant patients at doses up to 3,500 mg/day for up to two years. There was no correlation between the incidence of adverse effects and dose of MMF, and no overt nephrotoxicity, hepatotoxicity, or myelotoxicity was observed. In a multicenter study in patients with biopsy-proven renal allograft rejection, successful rescue (stabilization or improvement of renal function) was achieved with MMF in combination with maintenance doses of cyclosporine and prednisone in 69% of patients. This result suggested that MMF may be effective in the treatment of renal allograft rejection after transplantation. In a large multicenter trial, MMF in combination with cyclosporine and prednisone was superior to a standard immunosuppressive regimen including azathioprine. Taken together, the data indicate that MMF will be a valuable addition to the list of immunosuppressants available for the prevention and treatment of renal rejection after transplantation.