BACKGROUND AND PURPOSE: Atrial fibrillation (AF) is a risk factor for stroke, although it may not always be directly responsible for the stroke. On the other hand, cardiac arrhythmias and electrocardiographic changes have been reported after ischemic stroke and numerous other intracranial pathologies. We tested the hypothesis that some patients with acute stroke may develop transient AF as a consequence of the stroke. METHODS: This study was based on 1,661 patients with first-ever stroke consecutively hospitalized and prospectively included into the Lausanne Stroke Registry. "Recent AF" was defined as AF discovered at or after ("after-admission" AF) admission in patients without any previous history of AF. Populations with recent AF and after-admission AF were compared for AF evolution, risk factors, and lesion type and distribution with patients with previous history of AF (known AF) and with patients with another recognized cardiac source of embolism (cardioembolic). RESULTS: Twenty-four patients had recent AF on admission, and 17 developed it a few hours to 3 days after stroke. AF disappeared after a few days in 26 (63%; 94% of after-admission AF) patients. Stroke was a primary hematoma in 9.8% of patients with recent AF, 2.8% of patients with known AF, and 0.9% (p < 0.001) of patients with cardiac source of embolism. Parietoinsular (32%) and brain stem (11%) involvement were more common in recent AF than in cardioembolic stroke in general (16.7% and 6.7%, respectively; p < 0.05). CONCLUSIONS: AF discovered after an acute stroke lasted no more than a few days, suggesting that it may have occurred as a consequence of the stroke. This possibility is emphasized by the significant predominance in patients with recent AF of primary hematoma, which cannot be caused by AF, and of parietoinsular and brain stem involvement, which are experimentally known as arrhythmogenic. This hypothesis should be considered in patients with acute stroke and previously unknown AF before therapeutic decisions are made.