BACKGROUND New-onset postoperative atrial fibrillation (POAF) is a common complication of cardiac surgery that has substantial effects on outcomes. The aim of this study is to analyze the risk factors in the pre, intra, and postoperative periods, and evaluate its impact on patients' outcome. METHODS In this prospective study, between March 2007 and February 2011, a total of 1254 patients with preoperative sinus rhythm who underwent open cardiac surgery were included of which 177 (13.6%) had developed POAF. Many clinical variables that are associated with the development of POAF, were evaluated. RESULTS The study population consisted of 1254 patients that 864 (68.9%) were male and 390 (31.1%) female, and average age was 55.1±15.7 years. POAF occurred in 171 (13.6%) of patients and most of them (68.4%) developed within the first two days after surgery. Multivariate logistic regression analysis was used to identify the following risk factors of POAF: Preoperative risk factors: age>50, smoking, Left ventricular hypertrophy, renal dysfunction, intraoperative risk factors: intraoperative inotrope use, valve surgery, atrial septal defect (ASD) surgery, bicaval cannulation, concomitant cardiac venting of pulmonary and aorta, longer cardiopulmonary time, longer cross-clamp time, postoperative use of inotropic agent after termination of cardiopulmonary bypass. CONCLUSIONS POAF is the most common arrhythmia after cardiac surgery and not only concerted effort should be performed to identify and to reduce the risk factors, but also effective treatment is necessary to prevent mortality and morbidity.