BACKGROUND The purpose of the present study was to investigate the association between preoperative heart rate variability and atrial fibrillation after off-pump coronary artery bypass graft surgery. METHODS Of 524 consecutive patients undergoing isolated coronary artery bypass surgery, 390 were retrospectively analyzed after excluding the following 134 cases: on-pump surgery (n = 6), emergency (n = 106), chronic atrial fibrillation (n = 17), and pacemaker rhythm (n = 5). The following time-domain factors of heart rate variability were calculated: standard deviation of all normal-to-normal QRS (SDNN) and square root of mean of sum of squares of differences between adjacent normal-to-normal QRS (RMSSD). RESULTS Atrial fibrillation occurred in 98 patients (25%) after surgery. Patients not having atrial fibrillation had significantly lower heart rate variability than did patients having atrial fibrillation, with median values of 91 versus 121 for SDNN and 19 versus 25 for RMSSD. Reduced heart rate variability was significantly associated with a lower risk of postoperative atrial fibrillation: the adjusted hazard ratio (95% confidence interval) was 0.29 (0.17 to 0.49) for SDNN 99 ms or less and 0.47 (0.30 to 0.74) for RMSSD 20 ms or less. The area under the receiver operating characteristic curves for SDNN and RMSSD as a predictor of postoperative atrial fibrillation was 0.764 and 0.696, respectively. CONCLUSIONS Reduced time-domain factors in preoperative 24-hour heart rate variability are independently associated with a lower risk of atrial fibrillation after off-pump coronary artery bypass surgery.