Thickening of the basement membrane zone (BMZ) is a characteristic feature of airway remodeling. The BMZ appears as three component layers: the laminas lucida, densa, and reticularis. The lamina reticularis of the BMZ is thickened in asthma, allergic rhinitis, eosinophil bronchitis and lung transplants. Collagen types I, III and V form heterogeneous fibers that account for the thickness of the BMZ. Proteoglycans are structural component of the BMZ responsible for many of its functions, in particular, trafficking of growth factors and cytokines between epithelial and mesenchymal cells. An important function of the BMZ is storage and regulation of fibroblast growth factor-2 (FGF-2). FGF-2 has been shown to be involved with normal growth and thickening of the BMZ. Treatment with corticosteroids reduces the width of the BMZ in asthmatics. The significance of BMZ thickening in airway function is not clear however, it may have a positive effect by physically protecting against airway narrowing and air trapping. Thickening of the BMZ may have a negative effect by influencing how the epithelial mesenchymal trophic unit functions. However at this point there are no studies showing abnormal trafficking of growth factors and cytokines due to thickening of the BMZ.