PURPOSE: To achieve pathologic correlation in areas of ground-glass attenuation (GGA) in usual interstitialpneumonia(UIP) and, using serial thin-section CT, to observe changes in those lesions. MATERIALS AND METHODS:CT-pathologic correlation was performed at 20 sites of GGA in 18 patients with UIP. Two chest radiologistsassessed serial CT scans, focusing particularly on areas of GGA where open lung biopsy had been performed.Pathologic score of inflammation vs. fibrosis was recorded by two independent lung pathologists. RESULTS: Oninitial CT, GGA appeared as an isolated manifestation at one site, mixed with irregular lines at six, and mixedwith both irregular lines and bronchiectasis/bronchiolectasis at thirteen. Pathologically, those areascorresponded respectively to areas of active inflammation, dominant inflammation, and dominant fibrosis. Theextent of GGA seen on follow-up CT decreased when it was an isolated finding (n=1), and at four of six sites (67%)where it was associated with irregular lines. At the remaining two sites, GGA persisted. GGA, when combined withirregular lines and bronchiectasis/bronchiolectasis, persisted (n=6) or progressed (n=7). CONCLUSION: GGA in UIP,when it appears combined with irregular lines and bronchiectasis/bronchiolectasis, pathologically denotes areas offibrosis and does not improve with treatment. Isolated areas of GGA or GGA associated with irregular lines appearto be inflammatory and improve with treatment.