CONTEXT: Atrial fibrillation is a common, but potentially preventable, complication following coronary artery bypass graft (CABG) surgery. OBJECTIVES: To assess the nature and consequences of atrial fibrillation after CABG surgery and to develop a comprehensive risk index that can better identify patients at risk for atrial fibrillation. DESIGN, SETTING, AND PARTICIPANTS: Prospective observational study of 4657 patients undergoing CABG surgery between November 1996 and June 2000 at 70 centers located within 17 countries, selected using a systematic sampling technique. From a derivation cohort of 3093 patients, associations between predictor variables and postoperative atrial fibrillation were identified to develop a risk model, which was assessed in a validation cohort of 1564 patients. MAIN OUTCOME MEASURE: New-onset atrial fibrillation after CABG surgery. RESULTS: A total of 1503 patients (32.3%) developed atrial fibrillation after CABG surgery. Postoperative atrial fibrillation was associated with subsequent greater resource use as well as with cognitive changes, renal dysfunction, and infection. Among patients in the derivation cohort, risk factors associated with atrial fibrillation were advanced age (odds ratio [OR] for 10-year increase, 1.75; 95% confidence interval [CI], 1.59-1.93); history of atrial fibrillation (OR, 2.11; 95% CI, 1.57-2.85) or chronic obstructive pulmonary disease (OR, 1.43; 95% CI, 1.09-1.87); valve surgery (OR, 1.74; 95% CI, 1.31-2.32); and postoperative withdrawal of a beta-blocker (OR, 1.91; 95% CI, 1.52-2.40) or an angiotensin-converting enzyme (ACE) inhibitor (OR 1.69; 95% CI, 1.38-2.08). Conversely, reduced risk was associated with postoperative administration of beta-blockers (OR, 0.32; 95% CI, 0.22-0.46), ACE inhibitors (OR, 0.62; 95% CI, 0.48-0.79), potassium supplementation (OR, 0.53; 95% CI, 0.42-0.68), and nonsteroidal anti-inflammatory drugs (OR, 0.49; 95% CI, 0.40-0.60). The resulting multivariable risk index had adequate discriminative power with an area under the receiver operating characteristic (ROC) curve of 0.77 in the validation sample. Forty-three percent (640/1503) of patients who had atrial fibrillation after CABG surgery experienced more than 1 episode of atrial fibrillation. Predictors of recurrent atrial fibrillation included older age, history of congestive heart failure, left ventricular hypertrophy, aortic atherosclerosis, bicaval venous cannulation, withdrawal of ACE inhibitor or beta-blocker therapy, and use of amiodarone or digoxin (area under the ROC curve of 0.66). Patients with recurrent atrial fibrillation had longer hospital stays and experienced greater infectious, renal, and neurological complications than those with a single episode. CONCLUSIONS: We have developed and validated models predicting the occurrence of atrial fibrillation after CABG surgery based on an analysis of a large multicenter international cohort. Our findings suggest that treatment with beta-blockers, ACE inhibitors, and/or nonsteroidal anti-inflammatory drugs may offer protection. Atrial fibrillation after CABG surgery is associated with important complications.