BACKGROUND: Atrial fibrillation (AF) is a frequent complication after percutaneous coronary intervention (PCI) and has been associated with increased in-hospital and long term mortality. The aim of the present study was to evaluate clinical characteristics of patients who complicated new-onset atrial fibrillation after acute coronary syndrome (ACS) treated by PCI. METHODS: We evaluated 11,093 consecutive patients with ACS who underwent PCI, enrolled in the Korea Acute Myocardial Infarction Registry from Jan. 2006 to Dec. 2007. Patients were categorized into two groups according to the development new-onset AF (group I: No AF, n=10,973; group II: AF, n=120). We analyzed baseline, clinical and coronary angiographic characteristics, and clinical outcomes were also compared. RESULTS: Overall incidence of new-onset AF was low (1.0%). Group II patients were older (p<0.001) and more frequent in female (p=0.006). There were significant differences in Killip classes (p<0.001), history of previous heart disease (p=0.029), cerebrovascular disease (p=0.001), diabetes mellitus (p=0.011) and renal disease (p=0.027). In coronary angiographic finding, Group II had higher rates of multivessel disease (p=0.024), left main disease (p<0.001) and lower success rate of PCI (p=0.005). During hospitalization, complications including cardiogenic shock, major bleeding, acute renal failure and sepsis were more frequently developed in group II (p<0.001). In-hospital mortality (3.7% vs. 18.3%, p<0.001) and 1-year MACE (20.3% vs. 41.5%, p<0.001) were higher in group II than group I. CONCLUSION: New-onset AF after PCI for ACS is significantly associated with poorer in hospital and long-term clinical outcomes.