Bronchiolitis obliterans syndrome (BOS) after lung transplantation is a disease of small airways that is currently graded according to a decline in forced expiratory volume in 1 second (FEV(1)) even in single lung transplant recipients in whom native diseased lung may influence lung physiology. The aim of this study was to evaluate the comparative changes in lung function and survival following the onset of BOS in patients with emphysema and patients with idiopathic pulmonary fibrosis (IPF) who have undergone single lung transplantation. We analyzed data from 31 single lung transplant recipients with emphysema and 25 with IPF who were at risk of BOS. There was no difference in the incidence of BOS between the 2 groups (10 patients with emphysema and 6 patients with IPF), but after the onset of BOS the patients with emphysema had a significantly greater median survival (18 months vs 8 months) despite a poorer mean FEV(1) (1.26 liter, 45% predicted vs 2.11 liter, 67% predicted) compared with the IPF group (p < 0.05) and this difference in lung function persisted at death (0.8 liter, 30% predicted vs 1.65 liter, 51% predicted) (p < 0.05). In summary the native lung physiology appears to influence lung function and therefore survival, and this may indicate that the classification of BOS should include disease-specific characteristics.