Obliterative bronchiolitis is a life-threatening non-infectious pulmonary complication of allogeneic hematopoietic stem cell transplantation (HSCT). It is characterized by the development of an obstructive abnormality which has been considered to be insensitive to bronchodilator treatments. However, this knowledge stems from measurements of forced expiratory volume in 1 s (FEV1) which is relatively insensitive to changes in small airway caliber. Moreover, it is known from studies in chronic obstructive pulmonary disease that symptoms improve after bronchodilator treatment even when FEV1 is minimally increased and correlate with the reduction of lung hyperinflation, which is the major consequence of severe expiratory airflow limitation. Therefore, measurements of airway caliber by parameters not affected by volume history and absolute lung volumes are preferable for assessing the effect of bronchodilator interventions.