Abstract Electrocardiographic changes in 31 patients with acute pericarditis have been analyzed and the results incorporated into a review of the evolution, differential diagnosis and pathogenesis of the electrocardiographic pattern in pericarditis. Pericardial effusion lowers the voltage and produces cyclic variations in amplitude of the electrocardiographic complexes. Pressure exerted by pericardial fluid or fibrin produces a “current of injury” which results in deviation of the S-T segment from the base line. Superficial myocarditis produces T wave abnormalities without change in the QTc interval. The magnitude and the direction of the S-T and T vectors and the evolution of the repolarization abnormalities help to differentiate the pericarditis pattern from the patterns of acute myocardial infarction.