OBJECTIVE: Differentiating cardiogenic pulmonary edema (CPE) from respiratory causes of dyspnea is particularly difficult in elderly patients. The aim of our study was to evaluate B-type natriuretic peptide (BNP) in patients older than 65 years presenting with acute dyspnea. DESIGN: Prospective study. SETTING: Medical emergency department of a 2000-bed urban teaching hospital. PATIENTS: Patients aged over 65 years presenting with acute dyspnea and a respiratory rate more than 25/min or a PaO(2) below 70 mmHg, SpO(2 )less than 92%, PaCO(2) higher than 45 mmHg with pH less than 7.35, were included. BNP levels, measured blind at admission were compared with the final diagnosis (CPE or no CPE) as defined by experts. INTERVENTION: None. MEASUREMENTS AND RESULTS: Three hundred eight patients (mean age of 80 years) were enrolled in the study. The median BNP was 575 pg/ml [95% confidence interval (CI): 410-898] in the CPE group (n=141) versus 75 pg/ml (95% CI: 59-98) in the no CPE group (n=167) (p<0.001). The best threshold value of BNP was 250 pg/ml, with a sensitivity and specificity for CPE of 0.78 (95% CI: 0.71-0.84) and 0.90 (95% CI: 0.84-0.93), respectively. The area under the ROC curve was 0.874+/-0.081 (p<0.001). The accuracy of BNP-assisted diagnosis was higher than that of the emergency physician (0.84 versus 0.77, p<0.05). CONCLUSION: Analysis of BNP is useful in elderly patients with acute dyspnea, but the threshold value is higher than that previously determined.