OBJECTIVES: To investigate the relationship between the plasma B-type natriuretic peptide (BNP) level and the occurrence of atrial fibrillation (AF) in nonobstructive hypertrophic cardiomyopathy (HCM) patients. METHODS: Patients (n=97) were classified into chronic AF (CAF; n=14), paroxysmal AF (PAF; n=18) and normal sinus rhythm (NSR; n=65) groups. The plasma BNP values were analyzed with logarithmic transformation. RESULTS: The PAF group showed significantly higher plasma BNP levels than the NSR group [mean (range; -l SD and +1 SD); 248.3 (143.5, 429.5) vs. 78.2 (27.9, 218.8 ng/L), p<0.0001]. The CAF group also showed significantly higher plasma BNP levels than the NSR group [291.1 (161.4, 524.8 ng/L), p<0.0001]. Multivariate analysis with other clinical factors selected association of PAF as one of the factors that increased the plasma BNP level. CONCLUSIONS: The present study indicated that plasma BNP level is clinically useful for identification of nonobstructive HCM patients who have a risk of PAF.