The diagnosis of heart failure in the outpatient setting can be difficult. A rapid assay for B-type natriuretic peptide (BNP) has been advocated for the diagnosis of heart failure, using a single cutoff of 100 pg/mL. BNP is produced by both the right and left cardiac ventricles and is elevated in a variety of conditions, including heart failure, pulmonary hypertension, cor pulmonale, pulmonary embolism, left ventricular hypertrophy, renal failure, circulatory overload, acute coronary syndromes, atrial fibrillation, lung cancer, and sepsis. This multitude of causes of BNP elevation imposes limits on its diagnostic use for heart failure. The literature on the use of BNP testing for diagnosis of heart failure is reviewed, and improved guidelines for its interpretation are suggested.