['Patients with atrial fibrillation (AF) and atrial thrombus are at high risk of thromboembolic events. We investigated whether BNP levels can serve as a biological marker of thrombus.', 'We prospectively enrolled patients with AF within 7days of an ischemic stroke and transient ischemic attack (TIA). We measured BNP levels in all patients while they underwent transesophageal echocardiography (TEE) and then assigned them to groups based on the presence (positive group) or absence (negative group) of left atrial thrombus. Factors associated with atrial thrombus were investigated using multivariate logistic regression analysis.', 'Of the 67 (male, n = 40; mean age, 76.5 ± 11.1 years) enrolled patients, 17 (25.4%) had left atrial thrombus. The incidence of hypertension was significantly higher in the positive, than in the negative group (88.2% vs. 58.0%, p = 0.020). The BNP level was also significantly higher in the positive, than in the negative group (median (interquartile range) 189.8 (141.4-473.2) vs. 117.9 (70.3-187.1) pg/ml, p=0.012). The optimal cut-off value, sensitivity, and specificity of BNP levels to distinguish the positive, from the negative group were 140.0 pg/ml, 76.5%, and 62.0%, respectively. Multivariate logistic regression analysis demonstrated that a BNP concentration of>140.0 pg/ml (odds ratio, 5.62; 95% CI, 1.39-22.66, p = 0.015) was an independent factor associated with thrombus.'] Is brain natriuretic peptide a marker associated with thrombus in stroke patients with atrial fibrillation?