Since B-type natriuretic peptide (BNP) concentration has been shown by recent studies to be elevated in patients presenting acute coronary syndrome (ACS) even in the absence of overt heart failure, other mechanisms for elevating plasma BNP (p-BNP) concentrations may be suggested to exist. We have studied the correlation between p-BNP level and the extent of myocardial ischemia (EMI) in non-ST elevation (NSTE) ACS and evaluated the BNP level as an objective marker of EMI. In 204 patients with NSTE ACS, we estimated the EMI by the echocardiographic wall motion score index (WMSI) and the coronary angiographic Gensini score. As the positive control group, 44 patients with stable angina were enrolled into the study. We compared their initial p-BNP levels with WMSI and the Gensini score. Additionally, peak troponin-T level was compared with p-BNP level in NSTE myocardial infarction (MI) patients. Using the multiple regression analysis, adjustments for age, left ventricle (LV) wall stress, LV mass amount and ejection fraction (EF) were made. Patients with LVEF < 45% or age > 75 years or underlying diseases that could elevate BNP levels were excluded from the study. P-BNP level was increased in NSTE ACS patients compared with stable angina patients (133.9 ± 87.4 vs. 12.2 ± 9.2 pg/ml, p < 0.05). P-BNP levels were found to correlate with WMSI and the Gensini score in unstable angina (r=0.519, p < 0.01; r=0.680, p < 0.01) and NSTEMI (r=0.716, p < 0.01; r=0.684, p < 0.01) patients, respectively. Additionally, p-BNP levels correlated with the peak troponin-T level in patients with NSTEMI (r=0.700, p < 0.01). P-BNP level might be a useful marker in the assessment of EMI.