Introduction: Atrial fibrillation (AF), a well-defined ischemic stroke (IS) risk factor whose prevalence increases with age, is associated with higher stroke severity. We aimed to evaluate stroke severity and hospital mortality in a nationally representative sample of AF-related IS patients. Methods: We utilized data from the National (Nationwide) Inpatient Sample databases from 2015 - 2018 using ICD-10 diagnostic codes to identify individuals with IS and comorbid AF. The NIHSS was used to characterize stroke severity in a subset of cases after 10/1/2016. Nonparametric statistics and logistic regression analyses were conducted to evaluate associations between AF and hospital death. Results: Of the 382,758 IS cases, 99,566 (26%) had comorbid AF. AF increased linearly with age, reaching at 47% of all hospitalized IS patients 85+ years of age or older (Figure). Higher age, male sex, white race, obesity, and higher median income were associated with comorbid AF, whereas diabetes, hypertension, tobacco use, and hyperlipidemia were associated with reduced odds of comorbid AF. While 5.8% of all IS patients died during hospitalization, mortality was increased nearly two-fold in those with AF (9.0% vs. 4.6%, p<.001). Among in-hospital deaths from IS, comorbid AF increased with age, present in 59% of those 85+ years of age or older (Figure). NIHSS, reported in 21% of patients, was higher in AF patients (mean NIHSS 6 vs. 9, p<.001). High NIHSS was the strongest independent predictor of hospital death. Conclusion: The burden of AF in a nationally representative sample of hospitalized IS patients is substantial, present in nearly 50% of the 85+ age group. AF-related IS is more severe and more likely to be fatal. As our population ages, the prevalence of AF will only increase. Understanding the severity and fatality of AF-related IS will have profound implications for health systems and may better facilitate anticipatory guidance and AF treatment.