BACKGROUND: The prognostic impact of atrial fibrillation (AF) in the setting of acute coronary syndrome (ACS) is controversial. Furthermore, there are limited real-world data on the management of ACS patients with history of AF. METHODS: The Global Registry of Acute Coronary Events (GRACE/GRACE2) and Canadian Registry of Acute Coronary Events (CANRACE) enrolled 14,285 patients across Canada between 1999 and 2008. Patients were stratified by the presence of history of AF. We compared clinical characteristics, medical therapies, cardiac procedures, and clinical outcomes between the 2 groups. RESULTS: Overall, 1333 of the enrolled patients (9.3%) had history of AF, of whom 51.5% presented with non-ST-segment elevation myocardial infarction, 29.5% with unstable angina, and 19.1% with ST-segment elevation myocardial infarction. Compared with the group without, patients with a history of AF less frequently received evidence-based antiplatelet and antithrombin therapies, left ventricle ejection fraction assessment, and coronary angiography (all P < 0.001); they also had higher unadjusted rates of in-hospital death, myocardial (re)infarction, and heart failure. However, in multivariable analysis, history of AF was not found to be independently associated with in-hospital mortality (adjusted odds ratio [OR] = 1.12; 95% confidence interval (CI), 0.73-1.73; P = 0.61) or death and/or myocardial reinfarction (adjusted OR = 1.15; 95% CI, 0.87-1.5; P = 0.34). CONCLUSIONS: History of AF is common among ACS patients. They received less evidence-based medical and invasive therapies than ACS patients without history of AF. History of AF is a negative independent predictor of in-hospital coronary angiography but was not found to be independently associated with adverse outcomes. All rights reserved.