Background: Although B-type natriuretic peptide (BNP) is being increasingly used in the diagnosis and treatment of congestive heart failure, its utility in patients with atrial fibrillation (AF) is unclear. The aim of this study was to characterize the importance of clinical and echocardiographic variables as determinants of plasma BNP levels, in patients with AF and preserved left ventricular (LV) systolic function. Methods: Plasma BNP levels were measured and comprehensive transthoracic echocardiography were performed in 47 subjects with sinus rhythm and 49 subjects with AF. Early mitral inflow velocity, (E) tissue Doppler derived diastolic mitral annular velocity, (E���) left atrial (LA) volume, and pulmonary artery systolic pressures were assessed. Measurements were averaged for 3 cardiac cycles in subjects with sinus rhythm and for 10 cardiac cycles in patients with AF. Results: Individuals with AF had higher BNP levels than those with sinus rhythm. (150 짹 114 vs. 49 짹 61 pg/ml, p< 0.001) Whereas the presence of AF was not found to be an Independent determinant of the BNP level, the LA volume index (r=0.63, p< 0.001), the pulmonary artery systolic pressure (r=0.45, p=0.006), and the E/E��� (r=0.36, p=0.04) were found to be significantly correlated with the BNP level. The correlations between BNP levels and LA volume index (r=0.65, p=0.001) or E/E��� (r=0.38, p=0.03) were not altered when subjects with sinus rhythm were removed. Conclusions: The BNP levels were found to be independently correlated with LA volume indices and tissue Doppler-derived E/E��� values in patients with AF and preserved LV systolic function. These results indicate that BNP levels may reflect early LV dysfunction and LA enlargement in this population.