After allogeneic bone marrow transplantation for chronic myelogenous leukemia, spontaneous pneumomediastinum and subcutaneous emphysema developed in a patient with bronchiolitis obliterans. Computed tomography scanning of the chest failed to reveal the cause. There was no evidence of a pulmonary process, pneumothorax, or pharyngeal or upper airway leak. Despite the alarming appearance of the patient, conservative management, including high-flow oxygen, resulted in resolution of the pneumomediastinum and subcutaneous emphysema. The cause of pneumomediastinum and subcutaneous emphysema in bone marrow transplant recipients is discussed.