The combination of tacrolimus (TAC) and mycophenolate mofetil (MMF) is frequently used for immunosuppression after organ transplantation (Tx), but the pharmacokinetics and interactions between the two drugs are poorly elucidated. We describe here the increase of TAC trough levels during MMF-induced diarrhea in a 8-yr-old boy after kidney Tx. Early dose reduction of TAC, together with short-term monitoring of TAC trough levels in the presence of diarrhea, is recommended.