BACKGROUND: In asthma, airway obstruction is usually reversible, either spontaneously or with treatment However, the responses to treatment are variable and some patients show an irreversible component of airflow obstruction. This may be associated with structural changes in the airway. To assess the relationship between the difference in airway reversibility and structural changes, we evaluated the HRCT findings and pulmonary function test. Methods : We studied 40 asthmatic subjects who had had acute exacerbation of symptoms and had showed normal chest X-ray findings. They had monitered PEFR daily, and had performed PFT and HRCT within three days after initiation of treatment According to serial PEFR, they were grouped into 3 categories (Group 0; initial PEFR was within normal limit, Group 1; revealed increment of 30% in PEFR within 3 days after initiation of treatment, Group 2 ; revealed within 2 weeks) and then grouped again into 4 (Group 0, Group 3; reached to normal value of PEFR within 3 days after initiation of treatment, Group 4 ; within 2 weeks, Group 5; not within 2 weeks). RESULTS: (1) Age in Group 0 was significantly lower than other groups(p<0.05), but there was no significance in other groups. (2) Duration of asthma in Group 2 was significantly longer than Group 0, l(p<0.05). (3) FVC(%) and FEV1(%) were significantly decreased with delayed response to the treatment (p<0.05). (4) FEV1/FVC(%) in Group 1, 2 were significantly lower than Group 0(p< 0.05). FEV1/FVC(%) in Group 5 was significantly lower than Group 0,3,4 (p<0.05). (5) Air trapping was increased significantly with delayed response to the treatment (p<0.05). (6) Mucus impaction in Group 0 was significantly larger than Group 1, 2 (p<0.05). FEV1/FVC(%) in Group 0,4,5 were significantly larger than Group 3 (p<0.05). CONCLUSION: Difference in reversibility of airway obstruction was associated with age, duration of asthma and severity of initial airflow obstruction. There was no definite difference in HRCT findings in asthma.