The purpose of this study was to investigate the relationship between the integrated backscatter (IBS) and atrial fibrosis, with and without atrial fibrillation (AF), in patients who are undergoing coronary bypass grafting (CABG).A total of 74 patients (18 preoperative AF, 56 sinus rhythm [SR]) with coronary artery disease undergoing CABG were included. The IBS of the left atrium (LA) posterior wall was acquired from transthoracic echocardiographic examination before surgery. Samples from the LA appendage were collected after opening the pericardium and quantitative assessment of atrial fibrosis was performed with collagen volume fraction (CVF). Postoperative AF was monitored with electrocardiographic telemetry in-hospital.There was a positive relationship between age, IBS of the left atrial posterior wall indexed by pericardium (IBS%), and CVF by multivariate linear regression analysis (r = 0.612, p = 0.034; r = 0.887, p < 0.001 respectively). The value of IBS%, LA dimension (LAD), and CVF values were higher in patients with preoperative AF than in patients with preoperative SR (p < 0.001, p < 0.01, p < 0.001, respectively). The value of age, IBS%, and CVF in patients with postoperative AF were higher than in patients without postoperative AF (p = 0.029, p < 0.001, p = 0.001 respectively).IBS of the LA posterior wall indexed by pericardium provides an objective quantitative measure of atrial fibrosis and correlates with postoperative AF in patients undergoing CABG surgery.Copyright 2009 Wiley Periodicals, Inc.