Pulmonary function tests and histology of a patient with bronchiolitis obliterans after hematopoietic stem cell transplantation (HSCT). Flow-volume loop of a forced vital capacity maneuver. Positive values represent expiration, negative values represent inspiration. The trace moves clockwise for expiration followed by inspiration. A: normal pulmonary function at one year after HSCT. B: abnormal pulmonary function one year later, with expiration prolongation, decreased forced expiratory volume at one second (FEV₁, 44% of the predicted value). C: histology of bronchiolitis obliterans showing dense concentric peribronchial fibrosis with luminal obliteration, but a relative paucity of inflammatory cells. Note the relative sparing of the alveolar spaces (with the courtesy of Prof. Stephan Dirnhofer, University Hospital, Basel, Switzerland)