The small airway disease in asthma is characterized by airway wall thickening associated with eosinophilic inflammation and hypervascularity. In our study, predicted forced expiratory volume in 1 s (% FEV1) correlated with the vascularity in the inner layers of large airways but not for small airways. High-resolution computed tomography (CT) scans of 0.5 mm collimation during acute mild exacerbations revealed mucus plugging, air-space nodules and ground-glass opacities. Mean lung density on CT in acute exacerbations was significantly increased compared with that in remission. These results suggest that the involvement of the small airways and lung parenchyma would be increased during exacerbations. Pathologically, eosinophils were significantly reduced by treatment with chlorofluorocarbon− beclomethasone dipropionate (BDP) in the large airways, but not in the small airways. New fine-particle inhaled corticosteroids (ICS), such as hydrofluoroalkane-BDP, can reach the small airways and lung parenchyma in asthmatic patients. From the results of peak inspiratory flow (PIF) through each dry powder inhaler in Japanese people, measurement of PIF should be recommended before the use of dry powder inhaler. In the present review, we address small airway disease in asthmatic patients using pathological and radiological methods and discuss the critical problems of current ICS therapy.