B-type natriuretic peptide (BNP) levels have been shown to be elevated in patients with paroxysmal atrial fibrillation (PAF); however, the underlying mechanisms have not been fully elucidated. Earlier, we reported that an increase in the augmentation index (AI), which is an index of wave reflection and arterial stiffness, is associated with PAF. In this study, we investigate the relationship between the BNP level and AI in patients with PAF. We enrolled 92 patients with a history of PAF and 90 age- and gender-matched individuals without PAF. AI was calculated using applanation tonometry of the radial artery when all patients were on sinus rhythm. Plasma BNP levels were measured simultaneously. An arterial stiffness parameter, the cardio-ankle vascular index (CAVI), was also evaluated. The increased AI in patients with PAF correlated with the elevation of the BNP level (r=0.47, P<0.01). When PAF patients were classified into tertiles on the basis of the BNP level, the left atrial volume index, left ventricular mass index, AI and CAVI increased, and mitral annular e' velocity (e'), as an index of left ventricular diastolic pressure, decreased with BNP tertiles. AI was also associated with e' and left ventricular mass index. Multiple regression analysis showed that the AI in PAF patients independently correlated with BNP levels. This study showed that AI was an independent correlate of the BNP level in PAF patients. Left ventricular diastolic dysfunction, which linked to an increase in arterial stiffness, may be involved in the elevated BNP level.