Shortness of breath is a common cause of consultation in the emergency unit. Therefore, it is essential to diagnose cases of cardiac failure. This may be difficult in some cases. The authors set out to assess the value of measuring brain natiuretic peptide in this context. Brain natiuretic peptide (BNP) was measured by an ultrafast method (Biosite/BMD) on arrival of 125 patients to the emergency unit. The results were then compared with the diagnoses made in the emergency unit and those of the hospital discharge summary. Nearly 18% of patients were wrongly classified in the emergency room; 1/3 were falsely diagnosed as cardiac failure and 2/3 were not recognised initially as having cardiac failure. In 90% of patients, in particular in the group wrongly considered as not having cardiac failure, BNP measurement could have helped correct the mistake. The optimal threshold value of BNP for diagnosis of cardiac failure in this study was 300 pg/mL, with positive and negative predictive values of 92.4 and 90.2%, respectively.