Sirolimus (rapamycin) is an immunosuppressant agent approved in 1999 for use in patients after renal transplantation. It is a macrocyclic triene antibiotic isolated from Streptomyces hygroscopicus. It acts by binding to an intracellular protein, FK-binding protein, resulting in the blockage of the mammalian target of rapamycin. This blockade causes downstream events that ultimately inhibit cell cycle progression from the G1 phase to the S phase in T lymphocytes, fibroblasts, and endothelial cells, which may explain the antifungal, immunosuppressive, and antiproliferative effects of sirolimus. In solid organ transplantation, sirolimus promotes tolerance, reduction of chronic allograft vasculopathy, and avoidance of toxic effects of other immunosuppressive drugs. In general, commonly described side effects of sirolimus have been dose-dependent thrombocytopenia, hyperlipidemia, rash, and edema. With increasing usage, there is now a growing body of literature reporting pulmonary toxicity of sirolimus, mostly lymphocytic interstitial pneumonitis without hemorrhage, fibrosing alveolitis, and a very few cases of DAH. It is believed, therefore, that in light of increased usage, pulmonary toxicity of sirolimus always should be considered because it can be easily misdiagnosed as opportunistic infection, coagulopathy, vasculitis, pulmonary edema, or neoplastic disorder, all of which are common in this subset of patients.