Rationale: Refractory lung function decline in association with recurrent pulmonary exacerbations is a common, yet poorly explained finding in cystic fibrosis (CF). To investigate the histopathologic mechanisms of pulmonary deterioration during adolescence and early adulthood, we reviewed clinicallyindicated lung biopsy specimens obtained during a period of persistent decline. Objectives: To determine if peribronchiolar remodeling is prominent in lung biopsy specimens obtained in adolescents with CF refractory to conventional therapy. Methods: Six adolescents with CF (mean age, 16.2 y; mean FEV1, 52% predicted at biopsy) with significant pulmonary deterioration over 12-24 months (mean FEV1 decline of 14% predicted/year) despite aggressive intervention underwent computed tomography imaging and ultimately lung biopsy to aid clinical management. In addition to routine clinical evaluation, histopathologic investigation included staining for transforming growth factor-β (TGF-β, a genetic modifier of CF lung disease), collagen deposition (a marker of fibrosis), elastin (to evaluate for bronchiectasis), and a-smooth muscle actin (to identify myofibroblasts). Measurements and Main Results: All computed tomography scans demonstrated a mix of bronchiectasis and hyperinflation that was variable across lung regions and within patients. Lung biopsy revealed significant peribronchiolar remodeling, particularly in patients with more advanced disease, with near complete obliteration of the peribronchiolar lumen (constrictive bronchiolitis). Myofibroblast differentiation (a TGF-β-dependent process) was prominent in specimens with significant airway remodeling. Conclusions: Constrictive bronchiolitis is widely present in the lung tissue of adolescents with CF with advanced disease and may contribute to impaired lung function that is refractory to conventional therapy (antibiotics, antiinflammatories, and mucolytics). TGF-β-dependent myofibroblast differentiation is prominent in areas of active fibrogenesis and may foster small airway remodeling in CF lung disease.