The aim of this study was to examine the relationship between the structural changes in large and small airways in asymptomatic asthmatics quantified by high-resolution computed tomography (HRCT) and airflow obstruction. The bronchial wall thickness at the trunk of the apical bronchus (B1) of the right upper lobe was used for assessment of the large airways. Air trapping, evaluated by the ratio of the average CT-determined values for the bilateral upper and lower lung segments at full expiration to that at full inspiration (E/I ratio), was used for assessment of the small airways. Measurements were obtained with a helical HRCT in 24 asymptomatic asthmatics followed by optimal treatment with inhaled and/or oral corticosteroids for >6 months. Prior (20-30 min) to the HRCT examination, all patients were given an inhaled bronchodilator. The ratio of airway wall thickness to outer diameter (T/D) and the percentage wall area (WA%) at the B1 bronchus and the E/I ratio were significantly greater for the 14 asthmatics with deficient reversible airflow obstruction (forced expiratory volume in one second (FEV1) <80% prediced or FEV1/forced vital capacity <70% after bronchodilator inhalation) than for the 10 asthmatics with normal spirometry and seven normal subjects. T/D, WA%, and E/I ratio showed significant negative correlations with FEV1 % pred after bronchodilator inhalation. The E/I ratio also showed significant positive correlations with T/D, WA%, and residual volume/total lung capacity. These findings suggest that, in spite of optimal treatment, structural changes in both large and small airways may simultaneously occur in asthmatics with deficient reversible airflow obstruction.