Background: Atrial fibrillation (AF) is the most commonly encountered clinical arrhythmia. However, The impact of AF in acute myocardial infarction (AMI) on future major cardiac adverse events is uncertain in Korean population. Methods: Between November 2005 and November 2007, 12,928 patients were registered in the Korean Acute Myocardial Infarction Registry (KAMIR). Patients were divided into 2 groups according to presence of AF: Group I (who had not AF, n=12,350) and Group II (who had AF, n=578). In-hospital mortality, mortality in 1 year, and overall mortality were compared between groups. Also, we examined independent predictors for mortality in AF groups. Results: The incidence of AF in AMI was 4.7%. AF group was significantly older (Group I: 62.5짹12.7, Group II: 68.5짹11.6, p<0.001) and more likely to have lower systolic blood pressure (Group I: 130.0짹30.2, Group II: 122.1짹31.7, p<0.001) and lower ejection fraction (Group I: 51.7짹12.6, Group II: 48.7짹13.1, p<0.001). There was no significant statistical differences in the prevalence of diabetes, hypertension, dyslipidemia, and angiographic findings (p>0.05). There were significant differences in failure rate in percutaneous coronary intervention (1.6% vs. 3.8%, p=0.004), In-hospital mortality (4.2% vs 10.8%, p<0.001), mortality in 12 months (4.8% vs 9.7%, p=0.002), and overall mortality (12.1% vs. 27.8%, p<0.001). However, AF was not a independent predictors in overall mortality. In multivaritate analysis, age (OR 1.094, CI: 1.047~1.142, p<0.001), blood pressure (OR 0.979, CI: 0.964~0.994), left ventricular ejection fraction (OR 0.931, CI: 0.900~0.965, p<0.001), percutaneous coronary intervention (OR 2.409, CI: 1.036~5.60, p=0.041) were independent predictors for overall mortality in AF with AMI patients. Conclusion: Although having poor prognosis, AF in AMI is not a independent predictor for mortality. Age, blood pressure, left ventricular ejection fraction, percutaneous coronary intervention are independent predictors in AF with AMI patients.