Acute pericarditis is among the differential diagnoses of patients presenting with acute chest pain to emergency department and ECG remains and important diagnostic modality during the acute phase of clinical presentation. The evolution of electrocardiographic manifestations of acute pericarditis has been described through four phases based on ST- segment and T wave changes. Stage I is most diagnostic and diffuse ST-segment elevation and PR-segment depression and can last a few hours to several days. Stage II is normalization of the ST and PR segments, and sometimes one or more leads lead or lag the others. Stage III is widespread T-wave inversions and Stage IV is normalization of the T waves with usually complete resolution of ECG changes. Orderly progress through stages has been regarded as characteristic and pathognomonic for acute pericarditis. Some stages may be absent if the process resolves too rapidly. The most reliable distinguishing feature between acute pericarditis and ST elevation myocardial infarction may be the ratio of ST-segment elevation to T-wave amplitude in lead V6. When this ratio exceeds 0.24, acute pericarditis is almost always present.