The aim of this study was to evaluate bronchial and lung abnormalities in patients suffering from moderate asthma as defined by international guidelines, with special attention to air trapping on CT in comparison with that detected in smoking and non-smoking normal subjects. Twenty-two patients classified as moderate asthma and control subjects including healthy volunteers, smokers (n = 10) or non-smokers (n = 12) were prospectively explored by high-resolution CT (HRCT) performed at suspended full inspiration and expiration. The same expiratory protocol was performed 15 min after inhalation of 200 microg of salbutamol. Patients underwent pulmonary function tests within the same week and bronchodilator response was assessed following inhalation of salbutamol. Abnormalities of bronchi and lung parenchyma on inspiratory CT and air trapping on expiratory CT, in dependent and non-dependent areas, were assessed and scored semi-quantitatively by two independent observers. Comparison of score mean values between the different groups was performed using Mann-Whitney test and Spearman correlation between CT findings and pulmonary function tests were calculated. Mosaic perfusion was observed in 23% of asthmatics. Air-trapping scores were significantly higher in asthmatic patients than in non-smoking control subjects (p = 0.003), but not than in smokers. This difference was ascribed to non-dependent zones of the lung for which air-trapping scores were also higher in asthmatic patients (p = 0.003) and in smoking subjects (p = 0.004) than in normal controls. In the asthmatic group, a significant positive correlation was found between airways resistance and bronchial dilatation score (p = 0.01), and between small airways obstruction index and mosaic perfusion score (p = 0.05). In addition, both FEV1 and reversibility of small airways obstruction values correlated with air-trapping score (p = 0.03 and p = 0.007, respectively). No change could be detected in air-trapping score following salbutamol inhalation. Patients suffering from moderate asthma present mosaic perfusion and larger areas of air trapping than normal subjects, particularly in non-dependent areas of the lung. These lung abnormalities are related to small airways obstruction.