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COPD and bronchial asthma are chronic airway diseases with a different pathogenesis. Comparisons of differences in airway calibre by bronchial generation between these diseases and their importance to pulmonary function have not been fully studied. We investigated airway calibre and wall thickness in relation to pulmonary function in patients with asthma, COPD, asthma plus emphysema and normal subjects using CT.Sixty-three asthmatic patients, 46 COPD, 23 patients with asthma plus emphysema and 61 control subjects were studied cross-sectionally. We used a software with curved multiplanar reconstruction to measure airway dimensions from 3rd- to 6th-generation bronchi of the right lower posterior bronchus.Patients with COPD had increased wall thickness, but the airway was not narrow from the 3rd-(subsegmental) to 6th-generation bronchi. Mean bronchial inner diameter (Di) of 3rd- to 6th-generation bronchi in patients with asthma or asthma plus emphysema was smaller than that of COPD patients and normal subjects. Airway luminal area (Ai) of 5th-generation bronchi most closely correlated with pulmonary function in patients with stable asthma. Although Di was similar in patients with asthma and asthma plus emphysema, the Ai of 6th-generation bronchi correlated significantly with pulmonary function in patients with asthma plus emphysema.Airway calibre in asthma may be smaller than in COPD. Airflow limitations correlated more closely with peripheral Ai in patients with asthma plus emphysema than in patients with asthma alone.