Objectives: We sought to determine whether atrial tachycardia arising adjacent to the noncoronary aortic sinus (NCAS-AT) has distinctive atrial activation patterns in relation to targeted anatomic imaging. Background: The knowledge of atrial activation patterns of the NCAS-AT and its anatomic basis is very limited. Methods: Three-dimensional electroanatomic mapping was performed during NCAS-AT in 13 patients and during sequentially pacing from the noncoronary aortic sinus (NCAS) and the para-Hisian atrial area in 15 reference patients. The spatial relationship between the NCAS and the contiguous atria was studied in another 25 reference patients using computed tomography and in 12 human hearts using gross and microscopic anatomic examination. Results: During NCAS-AT, the para-Hisian area of the right atrium (RA) and the anteroseptal region of the left atrium were activated almost simultaneously. The initial activation area (within first 20 ms of atrial depolarization) was relatively wide (9.3 ± 2.6 cm2 on the RA map and 8.1 ± 2.1 cm 2 on the left atrium map). In reference patients, NCAS pacing reproduced a biatrial activation pattern of NCAS-AT and resulted in a wider initial activation area than the para-Hisian atrial pacing within first 20 ms of RA activation (10.1 ± 3.0 cm2 vs. 3.9±1.7 cm 2; p < 0.001). Anatomically, the wall of NCAS did not contain myocardial tissue, but was intimately related to the paraseptal regions of the atria such that the shortest distances from the NCAS to the RA and the left atrium were 1.7 ± 0.6 mm and 2.3 ± 0.9 mm (p < 0.01), respectively. Conclusions: NCAS-AT has distinct atrial activation patterns that can be explained in part by its spatial anatomic relationship with the atria. © 2010 American College of Cardiology Foundation.