Background: The circadian clock influences a number of cardiovascular physiological processes including the incidence of acute myocardial infarction. The aim of this study was to evaluate the relationship between clinical outcomes and circadian variations in ST elevation myocardial infarction (STEMI) and non-STEMI. Methods: Between November 2005 and July 2007, 3581 STEMI patients who admitted within 12 hours of symptom(door to balloon time ���6 hours) and 2732 non-STEMI patients within 1 week of symptom(door to balloon time���2 weeks) were enrolled from the Korea Acute Myocardial Infarction Registry(KAMIR).Time of MI symptom onset was divided into four 6-hour periods in phase with circadian rhythms. Clinical outcomes were compared among four groups(period I:midnight-6h,period II:6h-noon,period III:noon-18h, period IV:18h-midnight) and we evaluated in-hospital, 1month, 6month and 12month MACE. Results: There are no differences of baseline characteristics, angiographic findings in each groups.The highest incidence of the acute MI was between 6am and noon in STEMI or non-STEMI. in STEMI, Symtom to door time was the shortest in noon-18AM and in non-STEMI was 6AM-noon. These times are thought to good relative accessibility at work time. In STEMI patients, Inhospital total MACEs were not difference among four periods(period I:5.9% period II:5.4%,period III:4.4%, period IV:5.2%, p=0.562). There were no significant differences in 1 month MACE and 6 month MACE. 12 month total MACE were period I:14.6% period II:14.6%,period III:12%, period IV:14% p=0.321, respectively. In non-STEMI patients, Inhospital total MACEs were also not different statistically among four periods(period I:5.4% period II:5.7%,period III:4.2%, period IV:5.9%). There were no significant differences in 1 month MACE and 6 month MACE. 12 month total MACE were period I:15.8% period II:13.5%, period III:13.6%, period IV:14.6% p=0.321, respectively. Conclusions: A circadian difference in the onset of AMI did not influence the initial and longterm MACE.