Sirolimus (SRL) is a recently available immunosuppressive agent. SRL, is a macrolide isolated from Streptomyces hydroscopicus that, in complex with its cellular receptor, FK binding protein, potently inhibits downstream signaling by the mammalian target of rapamycin (mTOR). It has been shown to reduce the incidence of acute rejection episode after renal transplantation. SRL by itself does not seem to cause significant nephrotoxicity in most animals and human studies in normal conditions. However, when combined with calcineurin inhibitors, serum creatinine levels often increase. The mechanisms for the synergism of this side-effect are still discussed. Furthermore, recent clinical data have shown that the administration of SRL immediately after renal transplant delay the recovery from delayed graft function. This effect may be secondary to the inhibition of the proliferation of the renal tubular cells which is a normal process for tubular repair. Some experimental data have confirmed this hypothesis. Finally, in the long-term, SRL use has been associated with a significant increase of proteinuria which may in the long-term increase the risk of graft loss of cardio-vascular morbiomortality. For all these reasons, SRL nephrotoxicty has become an important issue after renal transplantation. The review will discuss the clinical and the experimental data regarding this complication, which has been underestimated.