There are more clinical conditions than are apparent on first consideration in the diagnosis of which a knowledge of the electrocardiographic patterns occurring in pericarditis may be helpful. It has been said often that the diagnosis of acute pericarditis is more often missed than made. When purulent pericarditis is present as a primary disease or as a complication of pleurisy or pulmonary disease it is especially important to make the diagnosis early so that surgical treatment may be instituted promptly. The postoperative occurrence of substernal or precordial pain, evidences of embarrassment of the circulation and a pericardial friction may provide a puzzling problem in differentiating between simple pericarditis and thrombosis of a coronary artery. The similarity between the electrocardiograms of the two conditions may lead to serious error unless the dissimilarities are appreciated. Pericarditis may also complicate acute rheumatic fever, cardiac wounds, pneumonia, uremia and occlusion of a coronary artery.