With an aging population and significant overlap of risk factors, the cohort of patients with acute coronary syndrome (ACS) and concomitant atrial fibrillation (AF) is a sizable and growing one, with implications on cardiac reserve, anticoagulation and antiplatelet therapies, and related complications. The present study uses a large national database to analyze the impact of AF on patients admitted with an ACS. We queried the 2012 to 2014 National Readmissions Database to identify patients admitted with an ACS using International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9 CM) codes. These patients were then subcategorized based on the presence or absence of AF. Analysis of their initial hospitalization, 30-day readmissions and healthcare utilization and the economic burden was performed. Among 1,558,205 patients with ACS, 270,966 (17.4%) were noted to have concomitant AF. At baseline, these patients were older and more likely female, with a significantly higher burden of comorbidities. Patients with AF had longer and more complicated index hospitalizations with significantly higher mortality rates (8.6% vs 4.6%). Coronary artery bypass graft was the preferred method of revascularization in patients with AF as compared to percutaneous coronary intervention. The 30-day readmissions were higher in the AF group (15.6 vs 10.8%), largely driven by noncardiac causes. This was associated with higher healthcare utilization with longer hospitalizations during index admission. Patients admitted with ACS and concomitant AF is a high-risk population with increased in-hospital complications and mortality, as well as short term readmissions. Coronary artery bypass graft appears favored over percutaneous coronary intervention for revascularization in patients with AF.