Background The origin of artifacts of the ascending aorta during transesophageal echocardiography has not been widely studied. This study was undertaken to investigate in vivo whether anatomic features could determine the appearance of artifacts. Methods and Results Transesophageal echocardiograms of 46 patients studied for suspected dissection with proven diagnosis (30 patients with and 16 without ascending aortic dissection) were reviewed. The incidence of artifacts was 46%, and it was similar in patients with and those without dissection (chi-square 0.516; P = not significant). Artifacts were located in the aortic lumen twice as far from the transducer as the atrial-aortic interface. The aortic diameter was larger in patients with than in those without artifacts (6.4 ± 1.1 vs 4.2 ± 0.9 cm, P < .001). An aortic diameter >5 cm and an atrial-aortic ratio ≤0.6 predicted the artifact appearance with good sensitivity, specificity, positive and negative predictive values, and diagnostic accuracy; these parameters reached a value of 100% by analysis only of patients without dissection. Conclusions An ascending aortic diameter >5.0 cm that exceeds the left atrial diameter with an atrial-aortic ratio ≤0.6 creates in vivo the conditions for the reverberation of the atrial-aortic interface within the aorta. Therefore, in patients with such anatomic features, artifacts must be suspected in the presence of linear structures within the aorta. (Am Heart J 1999;137:967-73.)