Sirolimus, a mammalian target of rapamycin (mTOR) inhibitor, is a potent immunosuppressant that is increasingly used in prevention and treatment of graft‐vs‐host disease (GVHD) in allogeneic hematopoietic stem cell transplant (HSCT) patients. However, data regarding its adverse effects in HSCT patients remain limited. We describe an 18‐year‐old HSCT patient with a history of invasive fungal infection, who developed pericardial effusion with cardiac tamponade and interstitial pneumonitis while receiving sirolimus for GVHD prophylaxis. Our case illustrates potentially life‐threatening complications of sirolimus use in allogeneic HSCT patients.