Sirolimus is a recently licensed immunosuppressant for organ transplantation that has been used as basic, adjuvant, or maintenance therapy for prevention of organ rejection. Well-known side effects of this agent are hyperlipidemia and bone marrow suppression. Interstitial pneumonitis is a relatively newly described adverse effect of the drug. A 43-year-old female recipient of a cadaveric kidney developed cough with blood-tinged sputum while receiving sirolimus immunosuppressive therapy. High-resolution computed tomographic scan and chest radiograph revealed interstitial infiltrations over bilateral lower lungs. No evidence of bacterial, fungal, mycobacterial, or viral infection was found and all tests for collagen vascular diseases were negative. Discontinuation of sirolimus resulted in a significant improvement of the lung disease.