Mounier-Kuhn syndrome is a congenital abnormality of the trachea and main bronchi characterized by atrophy or absence of elastic fibers and thinning of muscle, which allows the trachea and main bronchi to become flaccid and markedly dilated on inspiration with narrowing or collapse on expiration or cough. The abnormal airway dynamics and pooling of secretions in broad outpouchings of redundant musculomembranous tissue between the cartilaginous rings predispose to the development of chronic pulmonary suppuration, bronchiectasis, emphysema, and pulmonary fibrosis. A broad spectrum of clinical abnormalities has been documented in Mounier-Kuhn syndrome, ranging from minimal disease with good preservation of pulmonary function to progressive disease leading to respiratory failure and death. In the appropriate clinical setting, Mounier-Kuhn syndrome is diagnosed in women from chest radiographs when the transverse and sagittal diameters of the trachea exceed 21 mm and 23 mm, respectively, and when the transverse diameters of the right and left main bronchi exceed 19.8 mm and 17.4 mm, respectively. In men it is diagnosed when the transverse and sagittal diameters of the trachea exceed 25 mm and 27 mm, respectively, and when the transverse diameters of the right and left main bronchi exceed 21.1 mm and 18.4 mm, respectively. The diagnosis can be confirmed easily by computed tomography.