Though non-traumatic pneumopericardium is an unusual finding on roentgen examination of the chest, the diagnostic features are specific. In a review of the Hines Hospital records, only 2 cases were found. In one of these a gastric ulcer perforated into the subdiaphragmatic space and formed a fistulous tract, with extension into the pericardium. A peptic ulcer of the myocardium and pyo-pneumopericardium developed. In the second case an esophageal carcinoma, with necrosis, extended to the pericardium, producing pneumopericardium. Etiology Various mechanisms have been described for the production of pneumopericardium. Regardless of the underlying cause, infection commonly occurs, so that eventually pyo-pneumopericardium is present. Ludwig Pick (1), in 1894, published a monograph on perforation of the diaphragm by gastric ulcer as observed in 28 cases. In 10 of these, the pericardium was involved, and in 4 the myocardium was perforated. In some cases ulcerations of the posterior gastric wall had perforated into the left pleura. Most of the pericardial perforations were due to ulcers on the lesser curvature. In 1904, James (2) reviewed 38 cases of pneumopericardium and added 1 of his own. In 16 of the cases, trauma was responsible for the entrance of air into the pericardium. Other less common causes of pneumopericardium were perforations secondary to esophageal ulcer (3 cases), pulmonary tuberculosis (3 cases), esophageal carcinoma (2 cases), and gastric ulcer (2 cases). In single instances, pericardial perforation was attributable, respectively, to liver abscess, pneumothorax, lung abscess, communication with a bronchus in suppurative pericarditis, and direct pericardial paracentesis. Single cases were also included of pneumopericardium (or pyo-pneumopericardium) due to pericarditis without demonstrable connection with any outside air-containing structure, presumably caused by a gas-forming organism, and of ulceration of a necrotic hilar lymph node, possibly by the same mechanism, although this is not accurately described. James' case was one of esophageal perforation by a swallowed piece of bone. Tylecote (3) reported a case of gastric ulcer perforating into the heart. The patient was found dead, and autopsy showed an ulcer on the posterior part of the left ventricle communicating with the stomach. Monroe (4) cited 2 cases (reported by others) of rupture of the heart by gastric ulcer; Johannessen (5) and Rappert (6) each described a single similar case. Perforation of an amebic abscess of the left lobe of the liver into the pericardium was reported by Hartz (7). Outerbridge (8) published a case of pyogenic abscess of the left lobe of the liver perforating into the pericardium. In Gottesman's (9) case, pneumopericardium was secondary to radiation necrosis. A carcinoma of the fundus of the stomach was treated by the implantation of radium needles and pneumopericardium occurred two months later.