Lung toxicity is a rare but serious side effect of sirolimus, a mammalian target of rapamycin inhibitor used as an immunosuppressive agent in solid-organ transplant recipients. We report a case of 67-year-old man who had living-related renal transplant 12 years previously that was complicated by chronic allograft dysfunction. He presented with fever, cough, and shortness of breath, and his chest imaging showed bilateral patchy and ground glass opacities. Before symptoms of lung toxicity, the patient's sirolimus levels were in the range of high normal. Bronchoalveolar lavage ruled out infection, and a transbronchial biopsy was inconclusive. A fluorodeoxyglucose positron emission tomogram scan showed high uptake in the area of lung opacities with a standard uptake value of 4.7. His symptoms improved after he was switched from sirolimus to tacrolimus, and a thoracic computed tomography scan after 6 weeks showed complete resolution. Pulmonary toxicity should be considered in any patient on sirolimus with respiratory symptoms and opacities on chest imaging. The role of fluorodeoxyglucose positron emission tomogram scan in evaluation of sirolimus-induced lung toxicity has not been previously described, and this is the first report of this type of scan finding indicating intense inflammation in this condition.