Background: Diabetes mellitus (DM) is well known as a cardiovascular risk factor. However, the impact of DM on clinical outcomes in patients with acute myocardial infarction (AMI) has not been fully established in especially Koreans. We evaluated the one-year major adverse cardiac events (MACE; defined as the composite of cardiac death, non-fatal infarction, re-intervention and bypass surgery) in diabetic AMI patients. Methods: Among a total of 6930 patients from Korea Acute Myocardial Infarction Registry (KAMIR) data, 1984 patients have the history of DM, whether or not to have treatment. Results: Mean age was higher and the history of hypertension and dyslipidemia, atypical symptoms on admission, multi-vessel disease, and left main disease were more prevalent in DM group. However, male gender, current smoker, and ST-segment elevation MI were more prevalent in Non-DM group. Percutaneous coronary intervention (PCI) was performed in 83.5 % of total patients and the rate of successful PCI was not different between two groups (93.8 % vs. 93.2 %, p=0.458). DM group have significantly higher incidence of in-hospital mortality and one-year MACE (5.8 % vs. 9.8 % and 14.7 % vs. 22.4 %, p<0.001, respectively). In multi-variate analysis, age, creatinine clearance, three vessel disease, ejection fraction, and the history of DM were the independent predictors of one-year MACE [Hazard ratio (HR)=1.010; 95% CI=1.001-1.019, HR=0.995; 95% CI=0.992-0.999, HR=1.839; 95% CI=1.526-2.216, HR=0.972; 95% CI=0.965-0.978, HR=1.325; 95% CI=1.102-1.593, respectively]. Conclusions: Diabetic AMI patients seems to have more cardiovascular risk factor. In spite of reliable PCI results, DM was significantly associated with poor one-year outcomes in Korean AMI patients.