Pharmacologic interactions and absorption disturbances after transplantation may induce serologic fluctuation of immunosuppression and adversely affect outcome. We present data showing that trough levels of mycophenolic acid decreased by 50% during combined mycophenolate mofetil (MMF) and cyclosporine therapy compared with levels during combined MMF and tacrolimus therapy. In addition, cystic fibrosis patients required 30% higher doses of MMF to achieve the therapeutic levels of recipients without cystic fibrosis.