Background: Real-time monitoring of the esophagus during catheter ablation of atrial fibrillation (AF) may be necessary to avoid the potential risk of atrio-esophageal fistula, especially for the application of high energy power at the posterior left atrium (LA). Methods: In 16 patients with drug refractory AF (paroxysmal: 11, persistent: 5, age: 56.7짹8.5 years) undergoing catheter ablation under the deep sedation, we prospectively studied the course and contact of the esophagus in relation to posterior LA and the angiographic proximity to ablation lines encircling the pulmonary veins (PVs). Using a radiocontrast agent (Gastrografin, 20-30 cc) through the lumen of 16 Fr nasogastric tube, esophagograms were simultaneously performed with PVs angiography on left- and right-oblique fluoroscopic views, respectively. Posterior encircling of PVs was created under fluoroscopic monitoring of esophagogram to avoid energy application at the sites where the ablation catheter tip overlay both margins of the esophagus. Results: Under the deep sedation, significant migration of the esophagus was not observed during ablation. The esophageal course was in close proximity of the left PVs antrum in all. The distance from the ostium of left PVs to the esophagus was significantly shorter than that of right PVs (0.3짹5.7 mm vs. 24.1짹9.7 mm, P<0.001) on left-oblique view. In 14/16 patients (87.5%), mid to inferior aspects of left-sided posterior LA was in contact with the esophagus. To avoid direct contact of ablation catheter with the esophagus, we tailored posterior encircling lines of left PVs antrum toward more ostial site in 5/14 patients (35.7%). There was no evidence related to esophageal injury or fistula after ablation. Conclusion: The esophagus is in close proximity of the left PVs antrum and in contact with mid to inferior aspects of left-sided posterior LA. Real-time fluoroscopic monitoring of esophagogram using nasogastric tube filled with a radiocontrast agent during ablation is useful to avoid the life threatening complication such as atrio-esophageal fistula.