PURPOSE: To evaluate changes in lesions, as shown by follow-up high-resolution CT (HRCT) in patients withusual interstitial pneumonia (UIP), and the significance of ground-glass attenuation. MATERIALS AND METHODS: HRCTfindings in 23 patients with UIP were retrospectively reviewed. We quantitatively analysed the pattern and extentof lesions, as seen on HRCT, initially and during the most recent follow-up. We also compared pattern changesbetween group I (n=12; less than 25% of the initial extent of GGA) and group II (n=11, more than 26% of thisinitial extent). RESULTS: Initial HRCT findings included GGA and irregular linear density in all patients,honeycombing in 21 (91%), and consolidation in two patients (9%). During the most recent follow-up, HRCT showedthat the extent of GGA had decreased (p<0.01;average 7.4%), while that of fibrosis (sum of the extent of irregularlinear deasity and honeyeombing) had increased (p<0.01; average 8.8%). The average decreased extent of GGA was1.36% in group I and 14.09% in group II. The average increase in the extent of fibrosis was 11.3% in group I and6.4% in group II. The decreased extent of GGA was significantly statistically different between the two groups(p=0.035) and the initial extent of GGA correlated closely with the decreased extent of GGA seen on follow-up HRCT(r=0.5094, p=0.013). There was, however, no difference between the two groups in change in the extent of fibrosis(p>0.05). CONCLUSION: The greater extent of GGA, as seen on HRCT, of a patient with UIP shows much less fibrosison follow-up HRCT. In UIP, evaluation of the extent of GGA, as seen on initial HRCT, is a helpful indicator ofprognosis.