The aim of the present study was to use NTproBNP measurements in patients with AF of unknown time of onset and no signs or symptoms of heart failure to identify a subgroup with possibly recent onset of the arrhythmia. Detection of left atrial (LA) and LA appendage (LAA) thrombi or spontaneous echocardiographic contrast (SEC) by transoesophageal echocardiography (TEE) was used as a surrogate measure of safe cardioversion. This was based on the fact that the absence of LA/LAA thrombus is associated with safe cardioversion of AF, as demonstrated in the landmark Assessment of Cardioversion Using Transesophageal Echocardiography (ACUTE) study.