BACKGROUND Despite advances in the diagnosis and management of Acute Coronary Syndrome (ACS), Atrial Fibrillation (AF) remains a commonly encountered complication leading to adverse short-and long-term outcomes across the whole spectrum of ACS. At present, the underlying mechanisms of AF in myocardial ischaemia remain incompletely understood. This study evaluates the incidence and trends of new-onset AF in ACS, its impact on ACS management, the associated prognostic significance and its correlation with coronary artery disease. MATERIALS AND METHODS We analysed 100 consecutive patients with ACS between December 2014 and April 2016. Angiographic profile of patients with ACS with AF was compared. Cases were analysed with respect to Major Adverse Cardiac Events (MACE) and mortality during hospitalisation. RESULTS Out of 100 patients included in the study who developed AF, 96 of these had new-onset AF and 4 had previous AF. The AF group had a higher risk profile for MACE including those with hypertension history, prior infarct, hypotension on admission, tachycardia on presentation, a higher Killip class, non-ST elevation MIs and inferior infarcts. Moreover, patients with new-onset AF were more likely to have left main coronary artery disease versus patients with a prior AF history. In all 100 patients who underwent coronary angiogram, it revealed significant SVD in 40 cases, DVD in 32 cases, TVD in 20 cases, insignificant CAD in 4 cases and normal coronaries in 4 cases. The in-hospital mortality for new-onset AF, previous AF and non-AF patients were 14, 11.6 and 5.2% respectively. New onset AF persisted as an independent predictor for mortality in patients with significant coronary artery disease (involvement of Left Main, LAD, LCX, RCA). CONCLUSION AF is not an infrequent event during ACS. The presence of new onset AF with significant coronary artery disease is associated with nearly a 60% increase in mortality with MI patients. AF patients had a poorer prognosis with a higher incidence of malignant arrhythmias, reinfarction, ischaemic mitral regurgitation, heart failure, cardiogenic shock and inhospital mortality than non-AF patients. Our results suggest that the appearance of NAF during ACS should not be considered an isolated benign event, but a severe complication with prognosis implications.