Long-standing chronic severe asthma may be associated with structural changes of both proximal and distal airways. To assess these changes, high-resolution computed tomography (CT) using an Imatron Ultrafast CT scanner with 3 mm thick sections at 10 min intervals was performed at full inspiration. A limited set of CT sections was also obtained on full expiration. Twenty-four chronic severe asthmatic patients (age, 47.0 +/- 2.4 years; FEV1, 56.5% +/- 4.1% of predicted) were studied. The scans were assessed independently by two radiologists. While concomitant dilatation and thickening of intrapulmonary airways were observed in 12 patients, air trapping as assessed by areas of increased lucency on expiratory scans was noted in 20. The mean expiratory-to-inspiratory cross-sectional area (Exp/Ins) was 75.9% +/- 2.0%, compared with 44.6% +/- 1.0% in historical non-asthmatic subjects. FEV1 (% predicted) correlated with Exp/Ins and with CT features of air trapping (both rs = 0.60; P < 0.001) but not with airway dilatation or thickening. Our data indicate the presence of peripheral airways obstruction in chronic severe asthma, which may be one of the underlying reasons for increased severity of asthma.