The computed tomographic (CT) appearance of lobar collapse has yet to be defined. In an attempt to determine the characteristic appearance of collapse 95 cases were reviewed retrospectively in a wide variety of clinical settings over a 3 year period ending January 1983. In this report 38 cases of lobar collapse secondary to endobronchial occlusion are analyzed; the appearance of collapse without endobronchial obstruction forms the basis of a subsequent report. Computed tomography was accurate in determining the site of bronchial occlusion in all cases. In 36 of 38 cases collapse was caused by endobronchial tumors, including bronchogenic carcinoma, bronchial carcinoids, endobronchial metastases, and lymphoma. Differentiation between these tumors was not feasible with CT. Most cases of collapse were caused by central tumor. In those cases in which a bolus of contrast material was used differentiation between tumor mass and collapsed pulmonary parenchyma was possible. Two of 38 cases were found to have benign bronchial occlusion. In one case a mucous plug obstructing the left lower lobe bronchus was accurately defined. In another case a bronchial stricture occluded the right lower lobe bronchus. This represented the only false positive case in this series. It is concluded that CT is an accurate means for establishing the diagnosis of endobronchial obstruction. In most cases the diagnosis of neoplasia was possible, provided a bolus of contrast material was used to define tumor mass. The potential role of CT in evaluating patients with lobar collapse is discussed.