Objective: The role of coagulation parameters left atrial thrombus formation in atrial fibrillation has not been investigated before. We aimed to investigate the association between the beta-fibrinogen gene polymorphism or glycoprotein IIIa gene polymorphism and presence of left atrial (LA) thrombus or spontaneous echo contrast (SEC) in patients with atrial fibrillation (AF). Methods: Forty-seven patients with AF, in whom transesophageal echocardiography was performed, were included to this cross-sectional observational study. Patients were divided in two groups; those with LA thrombus (n=24) were assigned to group 1 and those without thrombus-in group 2 (n=23). DNA analysis was conducted to determine gene polymorphism in all patients. Mann-Whitney U test or Chi-square tests were used for statistical analysis Results: There were no significant differences between groups regarding to demographic and clinical characteristics. The frequency of beta-fibrinogen 455 G/A polymorphism was higher (37.5%) in group 1 as compared to group 2 (15.1%) but it did not reach statistical difference (p=0.23). When we added patients with severe SEC in the study group (patients with severe SEC and/or thrombus n=27) the difference (44.40%-10%) reached the statistical difference (p=0.01). Glycoprotein IIIa Pl A1/A2 polymorphism was not different between groups with (p=0.82) or without SEC (p=0.73). Conclusion: In patients with atrial fibrillation, beta-fibrinogen 455 G/A gene polymorphism is associated with the presence of left atrial thrombus and severe SEC. Beta-fibrinogen 455 G/A gene polymorphism may be a promising marker for the prediction of thromboembolism risk in patients with atrial fibrillation.