STUDY OBJECTIVE: To investigate associations of heart rate variability (HRV) measurements with postoperative atrial fibrillation (AF) in patients undergoing off-pump coronary surgery. DESIGN: Prospective, observational, exploratory study. SETTING: Large university-affiliated community medical center. PATIENTS: 50 patients undergoing off-pump coronary artery bypass grafting (CABG). INTERVENTIONS: Preoperative recording of electrocardiograms (ECGs) with subsequent off-line HRV analysis. Monitored ECG telemetry for 5 days after operation. MEASUREMENTS: Frequency and time domain analyses, and additional non-linear HRV determinations. Multivariate regression analysis of predictors of postoperative AF. MAIN RESULTS: AF occurred in 23 (46%) patients. Only the low to high-frequency ratio was associated with AF (2.35 +/- 1.8 v. 4.57 +/- 5.0 for patients without AF, P < 0.05). CONCLUSIONS: The off-pump approach does not protect against AF, and nonlinear HRV analyses provide little value in predicting AF after off-pump CABG.