Background-The proximity to vascular structures is a limiting factor during radiofrequency ablation. However, little or no attention has been given to the atrial arterial circulation during the development of atrial fibrillation (AF) catheter ablation techniques. Methods and Results-We examined the atrial arterial circulation in areas involved in AF ablation in 24 heart specimens by colored resin injection and careful dissection. The sinus node artery (SNA) arose from the circumflex artery in 42% of case; proximal to the LA appendage in 29%, crossing the left atrium (LA) anterior wall; and after the LA appendage in the remaining 13%, crossing the mitral isthmus and passing close to the left pulmonary veins (PVs), the LA roof, and the right superior PV. In 58%, the SNA arose from the right coronary artery. Major arteries (Ն1 mm in external diameter) were found in the mitral isthmus in 54%, at the LA roof in 54%, and at the LA anterior wall in 29%. Around the left PV ostia, there were areas with major arteries in up to 37% (at the roof and inferior segments) and around the right PV ostia in up to 29% (at the roof segment). Conclusions-Major atrial coronary arteries, including the SNA, were commonly found in the areas involved in AF ablation and could cause difficulties in obtaining transmural lesions and electric isolation or even lead to ischemic sinus node or atrial dysfunction. (Circ Arrhythm Electrophysiol. 2010;3:600-605.)Key Words: arteries Ⅲ atrium Ⅲ catheter ablation Ⅲ anatomy Ⅲ atrial fibrillation A trial fibrillation (AF) is the most common arrhythmia in clinical practice, with an estimated prevalence of 0.4% to 1% in the general population 1,2 and rising with age to 8% in patients Ͼ80 years. 3 Its pharmacological treatment has yielded suboptimal results, leading to a search for nonpharmacological options for therapy. 4 Clinical Perspective on p 605During the development of the surgical approaches for AF ablation, knowledge of the atrial coronary artery anatomy was essential, leading to important technical modifications and better results. Recognition of the importance of the sinus node artery (SNA) and its relationship to the areas of atrial surgical incisions contributed to refinement in surgical technique with the maze III procedure, 5 reducing sinus node and atrial dysfunction commonly found after the initial maze I surgery. Later studies with the atrial coronary anatomy in dogs led to the proposal of the radial incisions technique, 6 reducing even further the mechanic dysfunction observed during the follow-up of patients submitted to the initial techniques. 7 The earliest techniques of radiofrequency (RF) catheter ablation simulated the surgical maze procedure 8 or attempted to eliminate ectopic foci inside the pulmonary veins (PVs). 9 In more recent modifications, endocardial RF pulses are delivered circumferentially around the PV antrum to obtain electric isolation 10,11 associated with linear ablations in the left atrium (LA) and mitral isthmus in selected patients. 12 Despite the atrial coronar...