BACKGROUND: An early repolarization electrocardiographic (ER-ECG) pattern is caused by various pathophysiological conditions and is reported to be a predictor of life-threatening ventricular tachyarrhythmias. However, little evidence has been reported on the prevalence and significance of the ER-ECG pattern in acute myocarditis. This study aimed to investigate the prevalence and significance of the ER-ECG pattern and its mechanistic insight based on the cardiac magnetic resonance findings in patients with acute myocarditis. METHODS: Thirty patients (23 men; 39.2±19.1 years) with a diagnosis of acute myocarditis by a clinical presentation between March 2011 and April 2018 were retrospectively evaluated. The patients were divided into 2 groups depending on the presence of an ER-ECG pattern on admission. RESULTS: Nine cases had an ER-ECG pattern, which was defined as terminal QRS notching or slurring with an amplitude of >0.1 mV in at least 2 inferior or lateral leads (early repolarization [ER] group), whereas the remaining 21 cases had broad ST-segment elevation or pathological Q waves (non-ER group). The cardiac troponin T level was significantly higher in the non-ER group than ER group (3.2±4.3 versus 0.9±1.2 ng/mL; P=0.036). The ECG changes returned to baseline along with the normalization of the cardiac biomarkers. Nine of the 21 non-ER group patients, but none of the 9 ER group patients, developed a fulminant course and lethal ventricular tachyarrhythmias. T2-weighted cardiac magnetic resonance showed high-intensity signals over the entire transmural left ventricle in the non-ER group, whereas they were localized to the left ventricular epicardium in the ER group. CONCLUSIONS: The ER-ECG pattern in acute myocarditis was transient and reversible and was not associated with a worse prognosis. Inflammation/swelling localized to the left ventricular epicardium because of acute myocarditis might provide a mechanistic insight into the ER-ECG pattern.