Improvement in the ability to determine the specific cause of any given case of pulmonary edema would lead to more rapid and definitive treatment. "Wedge" pressures and measurements of cardiac output derived from Swan-Ganz catheterization assist in making this determination, but the procedure is invasive, expensive, associated with complications, and not infrequently inaccurate. A plain chest film is, however, almost invariably available in all patients with pulmonary edema, and as shown in this study, the cause of the edema can be determined with a high degree of accuracy by careful attention to certain radiographic features. An independent two-observer study was performed on 216 chest radiographs of 61 patients with cardiac disease, 30 with renal failure or overhydration, and 28 with capillary permeability edema. Three principal and seven ancillary features have been identified, all of which are statistically significant and permit the cause of the edema to be determined correctly in a high percentage of cases. The three principal features are distribution of pulmonary flow, distribution of pulmonary edema, and the width of the vascular pedicle. The ancillary features are pulmonary blood volume, peribronchial cuffing, septal lines, pleural effusions, air bronchograms, lung volume, and cardiac size. Differing constellations of these features occur, each of which is characteristic of a specific type of edema. Overall accuracy of diagnosis in this study ranged from 86% to 89%. The highest accuracy was obtained in distinguishing capillary permeability edema from all other varieties (91%), and the lowest in distinguishing chronic cardiac failure from renal failure (81%).