Kühnlein P, Mahnkopf C, Majersik JJ, et al. - Researchers tested the assumption that left atrial (LA) fibrosis, quantified using late-gadolinium-enhancement magnetic resonance imaging, predicts recurrent stroke or atrial fibrillation (AF) in patients with embolic stroke of undetermined source (ESUS). Atrial fibrosis was compared in healthy controls and patients with lacunar stroke, ESUS, and known AF with or without prior stroke. They prospectively followed patients with ESUS for the primary outcome of recurrent ischemic stroke, incident AF, or both. Two hundred three patients were recruited from three centers: 103 patients without AF (35 healthy controls, 15 with lacunar strokes, 53 with ESUS) and 100 patients with AF (50 with and 50 without prior stroke). Patients with ESUS have atrial fibrosis similar to that seen in AF. Atrial fibrosis ≥ 12% was linked to recurrent stroke, incident AF or both. Anticoagulation may benefit this subset of ESUS patients for secondary prevention of ischemic stroke.