PURPOSE: To evaluate on the basis of the pulmonary function test the correlation between the extent of air trapping on HRCT with the severity of airway obstruction and also to identify the prognostic effect of the extent of air trapping after treatment of asthma. MATERIALS AND METHODS: Thirty five patients with clinically diagnosed bronchial asthma and air trapping, as seen on HRCT, were included in this study. We quantitatively analysed on HRCT the extent of air trapping and then statistically compared this with the clinical parameters of the pulmonary function test. We classified the patients into two groups on the basis of the pulmonary function test and clinical status : Group 1(N=35), the total number of asthmatic patients ; Group 2(N=18), relatively stable asthmatics without acute asthmatic attack who showed FEV1 of more than 80% of the predicted value. Using the functional parameters of PEFR, one of the objective indications of improvement in airway obstruction, we also classified the patients into three groups on the basis of interval between treatment and clinical improvement. The result of this was as follows ; group 1, asymptomatic group(initial PEFR within normal limit, N=7) ; group 2, early responder(improvement of PEFR within three hospital days, N=18) ; group 3, late responder(improvement of PEFRwithin fourteen hospital days should there be a number here). Using HRCT, we then statistically analysed the differences between the three groups in the extent of air trapping. RESULTS: Among the total of 35 asthmatics, the extent of air trapping on HRCT showed significant correlation with the FEV1(r=-0.6161, p<0.001) and MEFR(r=-0.6012, p<0.001). Among the relatively stable asthmatics who showed FEV1 of more than 80% of the predicted value, MEFR(r=-0.7553, p<0.001) and FEF75(r=-0.7529, p=0.012) showed statistically significant correlation with the extent of air trapping on HRCT, but there was no significant corrlation between air trapping on HRCT and FEV1. In the three groups of asthmatics classified on the basis of interval between treatment and clinical improvement, the extents of air trapping on HRCT in the asymptomatic, early responder and late responder groups was as follows: (mean valuse) 9.31%, 21.50% and 27.89%, respectively ; these differences were statistically significant. CONCLUSION: The extent of air trapping on HRCT in asthmatics showed positive correlation with FEV1 and MEFR. Quantitative analysis of air trapping on HRCT also provides prognostic information about a patient's response to treatment.