To know the frequency of atrial fibrillation (AF) after coronary artery bypass graft surgery and the risk factors which predispose these patients to develop post operative AF. This prospective, observational, analytic study was carried out at national institute cardiovascular disease karachi, from January 2001 to January 2002. it included 200 patients undergoing elective or emergency coronary artery bypass graft (CABG) surgery without concurrent valvular heart disease, prior history of AF, ventricular arrhythmias or redo CABG surgery. All patients underwent CABG and were followed for three weeks for incidence of AF. patients who developed AF (group A) were compared with patients without AF (group B) for different variables. Univariate analysis was done using student's t-test for continuous variables and Chi squire test for categoric variable. AF is a common, but potentially preventable, complication following CABG surgery. The Advanced age, low ejection fraction, prolonged cross clamp and bypass time are the important risk factors which may predispose thesee patients to develop postoperative atrial fibrillation.