Atopy, allergy, or asthma rarely can complicate organ transplantation. We identified two patients who developed asthma following bone marrow transplantation. Neither patient had a documented history of allergy, atopy, or asthma, but their donors were human leukocyte antigen-identical siblings who had a history of asthma. Pulmonary function testing revealed decreased airflow. Investigation of the bronchial biopsy specimens revealed eosinophilia and histologic features that were compatible with asthma. No infectious pathogens were identified. Both patients received therapy with bronchodilators and inhaled corticosteroids with symptomatic improvement. A diagnosis of asthma should be entertained in the differential diagnosis of pulmonary complications in bone marrow transplant recipients.