Tracheal mucociliary clearance is governed by the interplay between ciliary activity and secretory functions of the epithelium. These functions are altered transiently by various physiologic and injurious stimuli, and chronically in asthma, chronic bronchitis, cystic fibrosis, and other forms of airway disease. In general, these conditions are associated with a depression in mucociliary clearance that may predispose to respiratory infections and the accumulation of secretions.