Attention has not previously been drawn in the radiologic literature to a visualization on the routine postero-anterior chest roentgenogram of the pericardial shadow along the left cardiac contour. The silhouette of the pericardium, normal or abnormal, becomes demonstrable in those instances in which there is sufficient deposition of subepicardial fat to afford a linear or curvilinear contrasting shadow of increased radiolucency between the density of the myocardial mass and the adjacent parietal pericardium. It must be understood that the explanation for the roentgen finding herein described is presented on a theoretical basis. Postmortem studies and roentgen-anatomic correlation will be the subject of a future report if results so warrant. Anatomico-Pathological Aspects The pericardium is described as a fibroserous sac in which the heart and the roots of the great vessels are contained (1). The parietal pericardium consists of a serous membrane composed of a single layer of flattened mesothelial cells resting on loose connective tissue, which connects it to an outer fibrous layer. The visceral pericardium, or epicardium, is lined by a single layer of polygonal cells which may be flat or cuboidal, depending on the state of contraction of the heart. Below this is a layer of connective tissue, the subepicardial layer of areolar tissue, connecting the epicardium to the myocardium and containing blood vessels, nerves, and varying amounts of fat (2). The pericardial cavity is merely a potential space, and in the normal state the serous layers of the visceral and parietal tunics are everywhere in contact and the adjacent surfaces are moistened by a slight amount of serous fluid. The mediastinal pleura lies in close apposition to the parietal pericardium and these structures are, in fact, fused above but separate at the level of the left lower pole of the heart. The separated pleural and pericardial layers, along with the diaphragm, invest a space filled with loose connective tissue and a variable amount of fat, a space variously referred to as the apical, or pericardial, or epicardial fat pad (3, 4). This extrapericardial collection of adipose and loose connective tissue so frequently seen on the roentgenogram in the left (and right) cardiophrenic angle is not to be confused with the subepicardial fat layer being described (Figs. 1 and 2). Gross inspection of the fresh human heart after removal from the pericardial sac, at the postmortem table, readily reveals the yellowish fatty tissues of the subepicardial layer upon its surface as seen through the closely applied and transparent overlying mesothelial cells. The amount of adipose tissue seen on the gross specimen shows considerable variation in different individuals, depending in part upon the general nutritional status preceding demise.