Mycophenolate mofetil (MMF) is now widely used in solid organ transplantation. MMF is rapidly converted to its active form, mycophenolic acid (MPA), upon reaching the systemic circulation. MPA is metabolized to its glucuronide metabolite, mycophenolic acid glucuronide (MPAG), by glucoronyl transferases in the liver and possibly elsewhere. MPAG is then excreted by the kidney. MPA is extensively and avidly bound to serum albumin. Previous studies have demonstrated that it is only the free (non-protein-bound) fraction of MPA that is available to exert its action. In vivo and in vitro studies demonstrate that renal insufficiency decreases the protein binding of MPA and increases free MPA concentrations. This decrease in protein binding seems to be caused both by the uremic state itself and by competition with the retained metabolite MPAG. The disposition of MPA in patients with severe renal impairment may be significantly affected by this change in protein binding.