Aim: To evaluate the factors affecting air trapping on expiratory CT, its correlation with pulmonary function tests and its value for discriminating pulmonary diseases. Method: A total of seventy-five patients 28 chronic obstructive pulmonary disease, 21 asthma, 17 interstitial lung disease, and 9 bronchiectasis patients were included in this study. All patients underwent inspiratory HRCT, expiratory HRCT, and pulmonary function tests. Expiratory scans were evaluated for the presence of air trapping. The cross-sectional area and the ratio of air-trapping was calculated. Smoking history, duration of illness were noted. Statistically a correlation between the level and extent of air trapping, its correlation with pulmonary function tests and factors affecting air trapping were evaluated. Result:Air trapping was detected in 59 patients. In ten of these patients there was no mosaic pattern on inspiratory images. Air trapping on expiratory images was mostly seen in asthma patients (7 out of 10). The level of air-trapping showed a good correlation with the extent of air trapping in general, but in asthma patients the level of air-trapping did not correlate with the extent of trapping. There was a good correlation between pulmonary function tests and the level and extent of air trapping. The duration of illness affected the extent of air-trapping. Conclusion: Expiratory images are effective for discriminating asthma from other obstructive lung disease. The level and extent of air trapping detected on expiratory images which are mainly affected by illness duration, are good predictors of pulmonary function tests.