To correlate areas of ground-glass attenuation at computed tomography (CT) with findings at open lung biopsy in chronic diffuse lung disease. The cases of 26 patients were included on the basis of (a) extensive areas of ground-glass attenuation as the predominant (n = 17) or exclusive (n = 9) abnormality at CT in the absence of honeycombing and (b) histologic evaluation at open lung biopsy. Severity of ground-glass attenuation was scored in the lobe sampled at biopsy, with separate analysis of associated lung changes. Correlation of CT with histologic findings at the 37 biopsy sites demonstrated that ground-glass attenuation corresponded to inflammation in 24 (65%) cases and to fibrosis in 13 (54%) cases. Eleven of the 13 patients (85%) with fibrosis had traction bronchiectasis or bronchiolectasis. These findings were not present in any of the patients with inflammation. In patients with chronic diffuse infiltrative lung disease, areas of ground-glass attenuation not associated with traction bronchiectasis or bronchiolectasis are a reliable indicator of inflammation.