Acute myocardial infarction(AMI) and unstable angina(UA) are part of a spectrum of clinical disease collectively identified as acute coronary syndrome (ACS) which is the most common proximate cause of sudden cardiac death. In a diagnosis of ACS, electrocardiogram(ECG) is still important. But, it is somewhat difficult to find out abnormal findings of ECG when it is taken in early phase of ACS. Therefore, it is necessary to record ECG on several times and to follow up even if there is no ECG abnormalities at first recording. The effective interventions for patients with ACS, particularly ST-elevation MI(STEMI), are extremely time-sensitive. It is imperative that we evaluate efficient risk stratification, and effective treatment of patients with ACS as quickly as possible.