Computed tomography (CT) is useful in evaluating bronchial and bronchiolar abnormalities. Common bronchial and bronchiolar abnormalities include bronchiectasis, asthma, bronchial obstruction, and various forms of bronchiolitis. Causes of bronchiectasis include cystic fibrosis, childhood viral infection, allergic bronchopulmonary aspergillosis, and pulmonary fibrosis. On CT scans, cystic fibrosis may manifest with bronchial wall thickening, bronchiectasis (usually cylindrical, with varicose and cystic forms seen in advanced cases), and mucus plugs in the bronchi. Allergic bronchopulmonary aspergillosis is characterized by central, varicose or cystic bronchiectasis, predominantly in the upper lobes, with mucoid impaction in the bronchi. Traction bronchiectasis may be seen on CT scans of pulmonary fibrosis. Asthma is characterized by bronchial wall thickening due to inflammation. Bronchial dilatation and air trapping may also be seen. Bronchiolitis obliterans is manifested by direct and indirect signs on CT scans; the former consist of centrilobular branching structures and nodules, and the latter consist of bronchiectasis and bronchiolectasis, mosaic perfusion, and air trapping. CT findings of bronchiolitis obliterans organizing pneumonia (also known as cryptogenic organizing pneumonia) include air-space consolidation and nodules, with occasional bronchial dilatation and pleural effusions.