Background and Objectives:Though it is well known that diabetes mellitus increase cardiovascular risk, there have been many controversies about the benefits of strict glycemic control in diabetic patients withacute myocardial infarction (AMI). We tried to assess whether diabetic patients with AMI show increased mortality and major adverse cardiovascular events (MACE) compared to non-diabetic patients and whether a strict glycemic control before event of AMI has any benefit for improving survival or preventing MACE in diabetic patients. Methods:We compared three distinct groups; Group 1 (non-diabetic patients), Group 2 (diabetic patients with HbA1C < 6.5%) and Group 3 (diabetic patients with HbA1C > 8.5%) in order to assess the associations between HbA1C levels and prognosis, using data from a nationwide sample of AMI patients in Korea (Korean Registry of Myocardial Infarction: KORMI) who had measurement of HbA1C (N = 2908). Results:Rate of 12 months mortality and MACE in diabetic patients with AMI was higher than those of non-diabetic patients. However, among diabetic populations, lower HbA1C group (Group 2) showed worse prognosis than higher HbA1C group(Group3). 12 months mortality rate of group1, group2 and group3 was assessed as 9.1% (P-value < 0.001) , 21.5% (P-value < 0.001) and 13.8% (P-value < 0.001), respectively. 12 months MACE of these three group was observed as 10.4% with P-value < 0.001 (Group1), 21.2% with P-value < 0.001(Group2), and 18.3% with P-value < 0.001(Group3). Cox proportional-hazards regression model showed significant increased mortality of Group 2 among three groups. Conclusion:In this large nationwide cohort of patients with diabetes presenting with AMI, we reaffirmed that diabetes caused increased mortality and MACE in AMI. However, according to the results of mortality and MACE among diabetes population, it was shown that long term strict glycemic control might cause worse prognosis, compared to the lenient to poor control group.