The chest radiograph of the patient with asthma is characterized by bronchial wall thickening and hyperinflation. On CT scanning of patients with asthma one may see airway wall thickening, thickened centrilobular structures, and focal or diffuse hyperlucency. Apparent bronchial dilation may be seen, but the diagnosis of bronchiectasis should be made with caution. Quantification of changes in the airway wall and lung parenchyma may be valuable in understanding the mechanisms of asthma and in evaluating the effects of treatment. Central bronchiectasis occurs in most, but not all, cases of ABPA. Patchy airspace opacity may be the sole radiologic manifestation of ABPA in some cases. Other fungi can rarely cause a similar syndrome. The challenge for the radiologist evaluating the images of a patient with asthma is to find complications, such as ABPA, or alternative diagnoses.