Mycophenolate mofetil (MMF) is a new immunosuppressant developed for the prevention and treatment of acute renal rejection after transplantation. Diarrhea is the most frequent side effect observed during treatment with MMF. Its pathogenic mechanisms remain unknown. We describe a case of severe diarrhea due to villous atrophy in a renal transplant recipient during treatment with MMF. The patient was free of symptoms before MMF. Villous atrophy disappeared a few months after MMF withdrawal.