BACKGROUND Pulmonary barotrauma is considered as complication of the use of positive-pressure ventilations. Nasal high-flow therapy is increasingly being used as an alternative to them. Nasal high-flow therapy rarely causes pulmonary barotrauma probably because airway pressures are lower when compared with invasive mechanical ventilation. Bronchiolitis obliterans syndrome after allogenic hematopoietic stem cell transplantation is triggered by an alloimmune response in the bronchioles and causes obstruction of the bronchioles. However, the threshold of additional positive pressure has not been determined in a patient with bronchiolitis obliterans syndrome. CASE REPORT A 14-year-old female patient with acute myeloid leukemia at high risk of recurrence received an allogeneic hematopoietic stem cell transplantation from an unrelated bone marrow donor. After engraftment, she developed acute graft-versus-host disease, followed by chronic graft-versus-host disease. Ten months post-transplantation, she developed bronchiolitis obliterans syndrome. She continued to receive nasal supplemental oxygen therapy for persistent dyspnea due to bronchiolitis obliterans syndrome. At month +25, hypercapnia was noted. Therefore, we carefully initiated nasal high-flow therapy for dyspnea and adjusted the oxygen dose to maintain 90% SpO2 to avoid life-threatening apnea. The flow rate was as low as 14 to 20 L/min to avoid the risk of barotrauma and the deterioration of air trapping. Unfortunately, she died of respiratory failure at month +31 post-transplantation. A lung autopsy revealed pulmonary barotrauma. CONCLUSIONS Nasal high-flow therapy, even at low flow rates, may cause fatal pulmonary barotrauma in bronchiolitis obliterans syndrome.