Computed tomography (CT) scans were compared with pathologic determinants of disease activity in 12 patients with idiopathic pulmonary fibrosis. The theory was that intraalveolar and interstitial cellularity would result in areas of opacification of air spaces on CT scans. All patients underwent open lung biopsy, and disease activity was assessed with a pathologic grading system. Seven patients had mild disease activity, five had moderate to marked disease activity. Opacification of air spaces was patchy in distribution, predominantly peripheral, and seen better on 1.5-mm rather than 10-mm collimation scans. Disease activity on CT scans was graded independently from 0 to 3 based on the presence and relative density of the areas of air space consolidation compared with the surrounding parenchyma. The pathologic score was significantly greater in the patients with high CT scores than in those with low CT scores (P = .001). Five patients with marked disease activity and five of seven patients with mild disease activity were correctly identified. CT may be useful in the assessment of disease activity in idiopathic pulmonary fibrosis.