Although the benefit of antithrombotic therapy for stroke prevention in atrial fibrillation (AF) is well recognized, its potential effect on stroke severity and outcome is less well established. Our objective was to examine the effect of preadmission antithrombotic therapy on stroke severity and outcome in patients with AF within a large comprehensive nationwide stroke survey. The data from consecutive patients with AF admitted with acute ischemic stroke or transient ischemic attack during a 2-month period were collected. The patients were categorized into 4 groups according to the use of preadmission antithrombotic therapy: no antithrombotic therapy, antiplatelet therapy, warfarin with an admission international normalized ratio (INR) <2 and INR of ≥2. Of 1,938 patients presenting with acute brain ischemia, 329 (17%) had AF. The age-adjusted rate of more severe stroke (baseline National Institutes of Health stroke scale score >5) stratified by antithrombotic therapy use was 70% for no antithrombotic therapy use, 55% for antiplatelet therapy use, 59% for warfarin with an INR <2, and 38% for warfarin with an INR of ≥2 (p = 0.01). Compared to warfarin therapy with an admission INR of ≥2, the adjusted odds ratio for more severe strokes was 4.0 (95% confidence interval [CI] 1.7 to 10.0) for no antithrombotic therapy, 2.2 (95% CI 1.0 to 9.4) for antiplatelet therapy, and 2.7 (95% CI 1.1 to 6.7) for warfarin therapy with an INR of <2. Similarly, graded associations of antithrombotic medication were observed with severe disability (modified Rankin Scale score >3) or death at discharge, with corresponding adjusted odds ratios of 4.1 (95% CI 1.8 to 9.9), 2.1 (95% CI 1.0 to 4.6), and 1.5 (95% CI 0.6 to 3.5), and 1-year mortality, with corresponding adjusted ORs of 2.4 (95% CI 0.9 to 6.7), 1.9 (95% CI 0.8 to 5.0), and 2.2 (95% CI 0.8 to 6.2). In conclusion, in addition to its established benefit for stroke prevention, effective anticoagulation therapy is associated with decreased stroke severity and better functional outcome and survival in patients with AF presenting with acute brain ischemia.