Pneumomediastinography has proved to be an excellent radiological method for studying the size, extent, and gross anatomical characteristics of mediastinal masses (1, 2, 5, 10, 11). It has also been employed successfully in estimating thymic size in autoimmune diseases and in studying the response of the thymus gland after steroid therapy (9, 11). To our knowledge there have been two European papers and a single American paper which discuss the results of pneumomediastinography in evaluating bronchogenic carcinoma (1, 6, 10). The purposes of this report are to describe the recent modifications of our original method of pneumomediastinography (2) and to relate our experience with this procedure in evaluating 64 instances of suspected bronchogenic carcinoma. We shall emphasize the rationale for and the technical details of successful pneumomediastinography and also describe radiological principles and criteria for the interpretation of pneumomediastinograms. Rationale Using injection technics, Marchand examined the subfascial spaces of the mediastinum in intact cadavers and demonstrated that a perivisceral layer of fascia invests the esophagus, tracheobronchial tree, aorta, and the major pulmonary vessels. He further showed that this fascial layer is continuous with the fibrous layer of the parietal pericardium. Beneath this “fascial envelope” lies an intercommunicating potential space filled with loose areolar tissue referred to as the “subfascial space” (7) (Fig. 1). Perhaps because of this anatomical arrangement of the fascial envelope, gas injected into the anterior mediastinum, but not directly into the subfascial space, will rarely dissect posteriorly into the deeper mediastinal structures. Conversely, gas placed into the subfascial space frequently dissects anteriorly to fill the anterior mediastinum. Our experience with pneumomediastinography and that of others indicates that the success of this procedure depends primarily upon being certain to inject the gas into the subfascial space, regardless of the injection technic employed (11). In studying the mediastinal extension of bronchogenic carcinoma delineation of all the mediastinum is especially important. Technics A: Methods. Condorelli, who pioneered in the development of pneumomediastinography, developed two basic percutaneous methods for the introduction of gas into the mediastinum (3, 4). One technic consists of the retrosternal injection of gas into the anterior mediastinum and the second method, the transtracheal approach, places the percutaneous needle through the posterior tracheal wall into the subfascial space about the trachea. Bariety et al. (1) and Sumerling and Irvine (11) use the transtracheal method when it is practical. Simecek and Holub (10) enter the subfascial space by perforating the tracheobronchial tree with an endobronchial instrument during bronchoscopy and inject the gas at this time (pertracheal insufflation).