Patients referred for cardiac surgery are increasingly older, with a higher prevalence of significant comorbidities and undergoing more extensive surgery. The aim of the study was to ascertain the incidence and presentation of postoperative atrial fibrillation in contemporary patients. A prospective single-center study. A tertiary academic center. Between January 2013 and December 2014, 1,356 consecutive patients (72% male, median age 68), including urgent and emergency cases, were analyzed. Preoperative paroxysmal atrial fibrillation was present in 163 (12%) and chronic in 156 (12%) patients. No interventions. Of the 1,164 patients without chronic atrial fibrillation and surviving at least 5 days, 599 (51%) developed postoperative atrial fibrillation, 43% after bypass, 55% after single valve, 74% after multiple valve, 66% after combined bypass and valve, and 54% after aortic procedures, p<0.001, respectively. In 29%, the duration of postoperative atrial fibrillation was less than 48 hours and did not recur, whereas in 71% the arrhythmia persisted for at least 48 hours or recurred during hospitalization. Patients with postoperative atrial fibrillation were significantly older, had a higher prevalence of previous atrial fibrillation and hypertension, larger left atrium, and required longer hospitalization with increased rates of reoperations and infectious complications. The authors report high, 10% to 20% greater than previously described, occurrence of postoperative atrial fibrillation in contemporary patients undergoing cardiac surgery. Most patients with postoperative atrial fibrillation experienced prolonged duration or recurrence of the arrhythmia. The type of surgery, advanced age, and previous atrial fibrillation were the most important risk factors.