In the emergency department (ED), patients do not present with a diagnosis. Rather they manifest a symptom for which the physician must determine the most probable cause. It is the sorting of possibly acutely fatal diagnoses that is unique to the practice of emergency medicine. Unfortunately, the historically available rapid diagnostic tools lack both sensitivity and specificity. This creates risk for the acutely ill patient in whom either a delayed or erroneous diagnosis can be associated with a startling increase in mortality. B-type natriuretic peptide (BNP) assessment in the ED is a valuable adjunct for increasing the accuracy of the initial clinical impression. This allows early outcome-improving interventions to proceed with greater certainty. Finally, as a prognostic marker indicative of illness severity, knowledge of BNP levels can improve the accuracy of disposition decisions.