Background: Atrial fibrillation (AF) is a cause of ischemic stroke and is associated with recurrence and with acute multi-vessel territory non lacunar stroke. Many cryptogenic strokes could be secondary to undiagnosed paroxystic AF. As left atrium (LA) dilatation is a risk factor of AF, we sought to determine if moderate or severe LA dilatation was associated with multiple vascular territory infarcts on magnetic resonance imaging (MRI) in patients with acute ischemic stroke without known AF. Normal LA diameter values are ≤4.0 cm for men and ≤3.8 for women. Methods: Patients who presented at our center between 2006 and 2011 with a diagnosis of acute ischemic stroke (AIS) or transient ischemick attack (TIA) who had a transthoracic echocardiography and a brain MRI were included. Patients with known or de novo AF diagnosed within 3 months of the event were excluded. The main outcome was the presence of acute multi-vessel territory infarct(s) on MRI.Results: 356 patients were included. The mean LA diameter was 37 mm in the control group (normal or mildly dilated LA) and 49 mm in those with moderately to severely enlarged LA (p<0.001). More patients in the LA dilatation group met the main outcome (20% vs 6%; OR 3.89; 95% CI 1.32-11.43; p=0.0014). Conclusions: LA dilatation (at least moderate) is associated with acute non lacunar multiple vascular territory infarcts on MRI in patients with AIS or TIA without known AF or a confirmed diagnosis of AF. Further studies are necessary to determine if this population might benefit from anticoagulation therapy.