Introduction: Long-term results of catheter ablation (CA) of persistent and long-standing persistent atrial fibrillation (AF) have been disappointing. The hybrid approach is one method of overcoming the limitations of CA. Aim: To present electrophysiological (EP) findings in patients undergoing hybrid surgical ablation to treat atrial fibrillation. Methods: Patients with persistent and long-standing persistent AF underwent thoracoscopic epicardial radiofrequency (RF) isolation of the pulmonary veins (PV) using AtriCure clamps followed by linear ablations in the left atrium (LA) using a linear pen and complemented by Marshall ligament disruption and left atrial appendage exclusion using an AtriClip device. As part of the study protocol, all patients underwent an EP study and RF ablation to eliminate any and all residual epicardial conduction gaps. Results: Seventy patients (49 male, median 63.5 years) were recruited for the study. The EP-phase of the study was performed 87 days (median) after the thoracoscopic ablation. At the time of the EP study, 76% of patients presented with sinus rhythm (SR), 7% with typical atrial flutter, 11% with atrial tachycardia, and 6% with AF. Left sided PVs were found to be isolated less often than right PVs (75.7% vs. 91.4%, p < 0.001). Complete isolation of all PVs was seen in 68.6% of patients. Complete isolation of the LA posterior wall was found in 22.9% of patients only. Conduction gaps in the left PVs were mostly found on the superior portion and on the ridge between the superior vein and LA appendage. In the right PVs, gaps were most often on the superior and posterior walls of the vessels. It was impossible to find typical gap locations on the LA roof connecting line since most patients required re-ablation of the whole line. The most common re-conduction site on the inferior line was found on the third of the line adjacent to the right inferior PV. Conclusion: Surgical epicardial ablation of the right sided PVs is significantly more successful compared to ablation of left-sided PVs. Linear epicardial ablations, completed under direct visual control are often unsuccessful. Our results may help cardiac surgeons focus on the typical sites of re-conduction and improve their results through additional RF applications. (C) 2017 The Czech Society of Cardiology. Published by Elsevier Sp. zo.o. All rights reserved.