['Atrial fibrillation is associated with greater baseline neurological impairment and worse outcomes following ischemic stroke. Previous studies suggest that greater volumes of more severe baseline hypoperfusion in patients with history of atrial fibrillation may explain this association. We further investigated this association by comparing patients with and without atrial fibrillation on initial examination following stroke using pooled multimodal magnetic resonance imaging and clinical data from the Echoplanar Imaging Thrombolytic Evaluation Trial and the Diffusion and Perfusion Imaging Evaluation for Understanding Stroke Evolution studies.', 'Echoplanar Imaging Thrombolytic Evaluation Trial was a trial of 101 ischemic stroke patients randomized to intravenous tissue plasminogen activator or placebo, and Diffusion and Perfusion Imaging Evaluation for Understanding Stroke Evolution was a prospective cohort of 74 ischemic stroke patients treated with intravenous tissue plasminogen activator at three to six hours following symptom onset. Patients underwent multimodal magnetic resonance imaging before treatment, at three to five days and three-months after stroke in Echoplanar Imaging Thrombolytic Evaluation Trial; before treatment, three to six hours after treatment and one-month after stroke in Diffusion and Perfusion Imaging Evaluation for Understanding Stroke Evolution. Patients were assessed with the National Institutes of Health Stroke Scale and the modified Rankin scale before treatment and at three-months after stroke. Patients were categorized into definite atrial fibrillation (present on initial examination), probable atrial fibrillation (history but no atrial fibrillation on initial examination), and no atrial fibrillation. Perfusion data were reprocessed with automated magnetic resonance imaging analysis software (RAPID, Stanford University, Stanford, CA, USA). Hypoperfusion volumes were defined using time to maximum delays in two-second increments from >4 to >8\u2009s. Hemorrhagic transformation was classified according to the European Cooperative Acute Stroke Studies criteria.', 'Of the 175 patients, 28 had definite atrial fibrillation, 30 probable atrial fibrillation, 111 no atrial fibrillation, and six were excluded due to insufficient imaging data. At baseline, patients with definite atrial fibrillation had more severe hypoperfusion (median time to maximum >8\u2009s, volume 48 vs. 29\u2009ml, P\u2009=\u20090.02) compared with patients with no atrial fibrillation. At outcome, patients with definite atrial fibrillation had greater infarct growth (median volume 47 vs. 8\u2009ml, P\u2009=\u20090.001), larger infarcts (median volume 75 vs. 23\u2009ml, P\u2009=\u20090.001), more frequent parenchymal hematoma grade hemorrhagic transformation (30% vs. 10%, P\u2009=\u20090.03), worse functional outcomes (median modified Rankin scale score 4 vs. 3, P\u2009=\u20090.03), and higher mortality (36% vs. 16%, P\u2009=\u20090·.3) compared with patients with no atrial fibrillation. Definite atrial fibrillation was independently associated with increased parenchymal hematoma (odds ratio\u2009=\u20096.05, 95% confidence interval 1.60-22.83) but not poor functional outcome (modified Rankin scale 3-6, odds ratio\u2009=\u20090.99, 95% confidence interval 0.35-2.80) or mortality (odds ratio\u2009=\u20092.54, 95% confidence interval 0.86-7.49) three-months following stroke, after adjusting for other baseline imbalances.'] Is worse stroke outcome in atrial fibrillation explained by more severe hypoperfusion , infarct growth , and hemorrhagic transformation?