Background Adequate rate control has been a concern in management of patients with atrial fibrillation(AF). Role of ventricular rate(VR) and its irregularity have not been well studied with respect to the elevation of NT-proBNP level. VR at which NT-proBNP level is lowest would be the optimal heart rate in patients with AF. Methods and Results In 85 patients with AF(69.8짹9.0 years, 43 men), NT-proBNP level, indices of VR irregularity(mean VR, standard deviation(SD) of VR, SD of successive differences and SD of 5-minute averages(SDAVR)) derived from 24-hour ambulatory ECG and transthoracic echocardiographic parameters(early mitral inflow velocity(e)/mitral annular velocity(e���), left atrial volume index and left ventricular ejection fraction(LVEF)) were measured. Potential determinants of NT-proBNP level were identified by univariate and multivariate linear analysis. Age, e/e��� and SDAVR were significant univariate predictors of NT-proBNP level, but mean VR did not. e/e��� and SDAVR remained as significant predictors after multivariate adjustment(棺=0.028, P=0.004, and 棺=-0.002, P=0.008, respectively). When the patients were stratified with the 33rd and 67th percentile values of e/e���(10.62 and 14.63, respectively), a significant positive correlation between mean VR and NT-proBNP level was present in patients with higher e/e쨈(N=57, 棺=0.007, P=0.043). Conclusions VR is not as important as LV diastolic function and VR irregularity in the elevation of NT-proBNP level in AF. VR control may be needed in patients with severe LV diastolic dysfunction. Our results support the findings of recently published RACE II trial. Key words : Atrial fibrillation, NT-proBNP