Objectives: The aim of this study is to assess the incidence of atrial fibrillation (AF) in acute coronary syndrome (ACS) patients and its prognostic impact on the in-hospitalmortality and outcome.Methodology: The studyincluded 500 patients (pts) admitted to Critical Care Department, Cairo University with diagnosis of ACS. Their average age was 57.2 ± 11.2years. Among them, males were 364 (72.2%). All pts were subjected to medicalhistory, clinical examination, ECG, Echocardiography, routine laboratory profile(included cardiac markers) and coronary angiography. Management and differenttherapeutic option were dectated according to guidelines (STEMI: primary PCI orthrombolytic therapy……etc). In hospital mortality and morbidity were recorded. Thestatistical package SPSS version 15 was used and logistic regression analysis was doneto test for significant predictors of new onset AF and mortality.Results: AF was recorded in 48 patients; new onset AF was found in 7% and previousAF was 2.6%. Pts with new onset AF (as compared to pts without AF) had higher mortality (22.9% VS 4.9%,p value=0.001), cardiac arrest (31.4% Vs 6%, pvalue<0.001), heart failure (68.6% VS 32.1%, p value<0.001), pulmonary oedema(11.4% VS 3.3%, p value=0.040), cardiogenic shock (40% VS 11.9%,p value<0.001),major bleeding (8.6% Vs 0.7%,p value=0.006) and hospital stay (10.2±10.1 VS6.2±4.2,p value=0.039). Age, sex, pulse, LA size, EF and prior history of stroke werefound to be independent predicators of new onset AF.Initial creatinine level andcardiogenic shock were found to be independent predicators of mortality .Conclusion:Pts with new onset AF had higher mortality and morbidity rate than pts withprevious AF or without AF.However when using regression analysis, new onset AF orprevious AF were not independent predicators of mortality.