Sirolimus (SRL) is a mammalian target of rapamycin inhibitor, which provides an immunosuppressive effect by inhibiting cell cycle progression. The encouraging results of combined SRL–cyclosporine therapy paved the way to further immunosuppressant combinations. Although SRL is relatively non-nephrotoxic when administered as monotherapy, it pharmacodynamically enhances the toxicity of calcineurin inhibitors. Other side effects may include hyperlipidemia and myelosuppression and less commonly wound healing impairment, proteinuria, edema and pneumonitis. Surprisingly, SRL also showed encouraging properties as an antiatherogenic and antineoplastic, opening a large spectrum of new potential applications. Whether SRL can be used safely over the long term with low doses of calcineurin inhibitors requires further study. The use of SRL as a corticosteroid-sparing agent also remains to be proven in controlled trials.