Introduction: The aim of this study was to evaluate the relationship of the left atrial size and the presence of thrombus or spontaneous echo-contrast in patients with atrial flutter/fibrillation addressed for direct electrical cardioversion or electrophysiological study. Methods: Trans-esophageal echocardiography was performed on 151 patients (mean age, 60.1 ± 10.4 years), 31.8% females, with atrial fibrillation or flutter undergoing pulmonary vein isolation, cavo-tricuspid isthmus ablation or electrical cardioversion. We searched for correlation between LA size and CHADS2 score. We also noted the value of the LA diameter that best predicted presence of thrombus in the left atrium or left appendage. Results: Thirty-five patients (19.3%) out of 151 presented left atrial thrombus or spontaneous echo-contrast at transesophageal echocardiography. There was a positive correlation between LA size and CHADS2 score (p = 0.006). ROC curve analysis established a value of 43 mm as cut-off to distinguish between patients with LA/ LA appendage thrombus or echo-contrast and patients without thrombus or echo-contrast. The sensitivity of this cut-off value is 80% and specificity 50%, with an area under curve of 0.658 (p = 0.005). Conclusions: Our ROC curve analysis showed that a value of 43 mm for left atrium diameter can differentiate with a sensitivity of 80% and specifi city of 50% between patients with and without LA/LA appendage thrombus/ spontaneous echo-contrast. LA diameter could indicate who should receive anticoagulation treatment and it should be part of the risk schemes assessing thromboembolic events in patients with atrial flutter/fibrillation.