Objective As shown previously in patients with new-onset atrial fibrillation (AF) without symptoms or signs of heart failure, N-terminal pro-brain natriuretic peptide (NTproBNP) increases rapidly, reaching a maximum within 24–36 h, and then decreases even if AF persists. A study was undertaken to use NTproBNP measurements in patients with AF of unknown time of onset to identify patients with presumed recent onset of the arrhythmia.