Background: Many epidemiologic studies have been reported the morning peak incidences of acute myocardial infarction (AMI). However, short and long-term clinical outcomes of circadian pattern have not been fully investigated in patients with AMI. Methods: From Korea Acute Myocardial Infarction Registry (KAMIR) database, we analyzed 4754 eligible patients (2793 STEMI, 1961 NSTEMI; age=63.2짹12.1 years) who had primary percutaneous coronary intervention (PCI) and early invasive PCI. The clinical impact of circadian variation was evaluated among four 6-hour intervals (12:00 midnight-6:00 AM, 6:00 AM-12:00 noon, 12:00 noon-6:00 PM, 6:00 PM-12:00 midnight). Various major adverse cardiac events (MACEs) at 12 months were evaluated. Results: There was a marked circadian variation with increased incidences of AMI during the second quarter of day (6:00 AM to 12:00 noon). In a concordance of previous studies, hypertension was more prevalent during the second quarter of day (STEMI: 45% vs. 51% vs. 46% vs. 46%, respectively; NSTEMI: 53% vs. 59% vs. 52% vs. 48%, respectively, p <0.05). Among four 6-hour-interval groups, symptom-to-door time, door-to-balloon time, procedural complexity, and successful rates of PCI were not significantly different. As shown in table, in-hospital mortality, MACEs were not significantly different for 12-month follow-up. Instead, old age, diabetes, and Killip class higher than II were independent factors for 12-month MACEs. Conclusion: Older age and additional comorbidities, but not the onset time of AMI, are likely to explain the deteriorating short-and long-term outcomes in patients with primary PCI and early invasive PCI.