Background Our objective was to investigate stroke severity and subsequent rate of mortality among patients with and without atrial fibrillation (AF). Contemporary data on stroke severity and prognosis in patients with AF are lacking. Methods and Results First-time ischemic stroke patients from the Danish Stroke Registry (January 2005-December 2016) were included in an observational study. Patients with AF were matched 1:1 by sex, age, calendar year, and CHA(2)DS(2)-VASc score with patients without AF. Stroke severity was determined by the Scandinavian Stroke Scale (0-58 points). The rate of death was estimated by Kaplan-Meier plots and multivariable Cox regression. Among 86 458 identified patients with stroke, 17 205 had AF. After matching, 14 662 patients with AF and 14 662 patients without AF were included (51.8% women; median age, 79.6 years [25th-75th percentile, 71.8-86.0]). More patients with AF had very severe stroke (0-14 points) than patients without AF (13.7% versus 7.9%, P