BackgroundWe prospectively investigated the role of sympathetic activation in the etiology of atrial fibrillation following coronary artery bypass grafting. Methods.: Continuous ambulatory monitoring was performed for 80 hours in 131 patients after coronary artery bypass grafting. Right atrial plasma norepinephrine levels were assessed preoperatively and every 4 hours for 48 hours postoperatively. Results.: Of the 131 patients, 50% (65) had development of atrial fibrillation and 36% (47) required treatment. Onset of atrial fibrillation was preceded by a significant increase in sinus rate and atrial ectopic activity. On multivariate logistic regression, elevated mean postoperative norepinephrine levels (5.78 ± 2.83 versus 3.57 ± 1.31 nmol/L; p < 0.0001), increased age (68.9 ± 5.7 versus 63.8 ± 8.7 years; p = 0.02), and decreased postoperative magnesium levels (0.79 ± 0.09 versus 0.83 ± 0.10 mmol/L; p = 0.02) were independently associated with the occurrence of atrial fibrillation. Conclusions.: Elevated norepinephrine levels suggest that sympathetic activation may be important in the pathogenesis of atrial fibrillation after coronary artery bypass grafting, and this underlines the importance of β-adrenoceptor blockade as prophylaxis.