Background: Acute myocarditis is an inflammatory heart muscle disease and it shows various clinical finding and prognosis. The objective of this study was to investigate the role of 12 lead electrocardiography in predicting poor outcome. Methods: Between August 2005 to December 2007, 68 patients (44.2짹16.9 years) were enrolled who met the expanded criteria for diagnosis of myocarditis. They were divided into two groups. [Group I: Excellent in-hospital outcomes (48 patients). Group II: Poor in-hospital outcomes, the patients have residual ventricular dysfunction, or death.(20 patients). Results: The baseline clinical characteristics between the two groups showed insignificant difference in Age (45.2짹18.1 vs. 41.9짹13.6, p=0.070), Sex (44.1% vs. 55.9%, p=0.130). In analysis of past history, there were no differences among two groups in hypertension (22.9% vs. 5.0%, p=0.077), diabetes mellitus (6.3% vs. 5.0%, p=0.842), and smoking (18.8% vs. 25.0%, p=0.561). In chief complaint, group I was more likely to have chest pain than other group (62.5% vs 35.0%, p=0.038). In 12 lead electrocardiography[ECG], poor outcomes group had longer QT interval [corrected QT > 480msec] (31.3% vs. 70.0%, p=0.003), more frequent arrhythmic event (47.9% vs. 90.0%, p=0.001), incidence of premature ventricular complex (14.6% vs. 45.0%, p=0.007), and incidence of ventricular tachycardia (10.4% vs. 30.0%, p=0.046). There are significant differences in left ventricular ejection fraction(LVEF)(42.7짹15.2% vs. 30.7짹12.6%, p=0.003). Conclusion: In patients with acute myocarditis , the presence of QTc interval prolongation (>480 ms), frequent arrhythmic event, ventricular tachycardia and/or premature ventricular complex in ECG are associated with poor prognosis and those findings can be useful in predicting prognosis.