Determining the cause of acute dyspnea in the emergency department (ED) is a frequent diagnostic challenge. The two most common causes of acute dyspnea are congestive heart failure (CHF) and pulmonary disease, conditions that can be difficult. to tell apart clinically. A correct diagnosis is usually based on physical examination, laboratory tests, and chest x-rays, all of which can yield nonspecific findings.(1) Brain natriuretic peptide (BNP) is a cardiac hormone with vasodilating, natriuretic, lusitropic, and renin-angiotensin-aldosterone inhibiting properties. BNP is secreted primarily by the myocardium in response to volume expansion and pressure overload. BNP and atrial natriuretic peptide (ANP) are activated by ventricular and atrial distension due to increased intracardiac pressure.