Background Plasma brain natriuretic peptide (BNP) is a sensitive and specific marker of left ventricular (LV) function. In the treatment of heart failure, especially in idiopathic dilated cardiomyopathy (IDC), β-blocker (BB) therapy has been established as a powerful strategy. The purpose of this study was to analyze relationships between changes in BNP level and LV function during BB therapy in patients with IDC. Methods In 30 patients with IDC who had already received conventional therapy, measurement for plasma BNP and norepinephrine levels and echocardiographic indices were evaluated before and 2 and 6 months after carvedilol in 21 patients and at baseline and after 6 months in 9 patients who did not receive carvedilol. Results After 6 months carvedilol treatment significantly improved LV end-diastolic dimension (LVEDD) (65 ± 8 to 61 ± 8 mm) and LV ejection fraction (LVEF) (34% ± 13% to 43% ± 12%) with intergroup differences; it significantly decreased BNP (127 ± 113 to 69 ± 92 pg/mL) with no intergroup difference; however, it did not decrease norepinephrine. BNP correlated strongly with LVEDD, LVEF, and LV mass index in carvedilol-treated patients. The degree of change in BNP correlated with that in LVEDD or LVEF 6 months after carvedilol. All 14 patients with decreased or unchanged BNP levels showed an increase in LVEF, and 4 of 7 with a rise in BNP had decreased or unchanged LVEF. According to receiver operating characteristic analysis, the optimal BNP levels for detecting LVEF <35% before and after carvedilol were 75.5 and 69 pg/mL, respectively. Conclusion: Plasma BNP levels may accurately reflect alteration in LV function and structure and can be used as a therapeutic indicator for risk stratification in patients with IDC during BB therapy. (Am Heart J 2001;141:925-32.)