Background: Atrial fibrillation is associated with increased but variable risk of stroke. Our aim was to validate the recently developed biomarker-based ABC (age, biomarkers [high-sensitivity troponin and N-terminal fragment B-type natriuretic peptide], and clinical history of prior stroke/transient ischemic attack)-stroke risk score and compare its performance with the CHA ₂ DS ₂ VASc and ATRIA (Anticoagulation and Risk Factors in Atrial Fibrillation) risk scores. Methods: The ABC-stroke score includes age, biomarkers (N-terminal fragment B-type natriuretic peptide and high-sensitivity cardiac troponin), and clinical history (prior stroke). This validation was based on 8356 patients, 16 137 person-years of follow-up, and 219 adjudicated stroke or systemic embolic events in anticoagulated patients with atrial fibrillation in the RE-LY study (Randomized Evaluation of Long-Term Anticoagulation Therapy). Levels of N-terminal fragment B-type natriuretic peptide, high-sensitivity cardiac troponin T (hs-cTnT), and high-sensitivity cardiac troponin I (hs-cTnI) were determined in plasma samples obtained at study entry. Results: The ABC-stroke score was well calibrated with 0.76 stroke/systemic embolic events per 100 person-years in the predefined low (<1%/y) risk group, 1.48 in the medium (1%–2%/y) risk group, and 2.60 in the high (>2%/y) risk group for the ABC-stroke score with hs-cTnT. Hazard ratios for stroke/systemic embolic events were 1.95 for medium- versus low-risk groups, and 3.44 for high- versus low-risk groups. ABC-stroke score achieved C indices of 0.65 with both hs-cTnT and hs-cTnI, in comparison with 0.60 for CHA ₂ DS ₂ VASc ( P =0.004 for hs-cTnT and P =0.022 hs-cTnI) and 0.61 for ATRIA scores ( P =0.005 hs-cTnT and P =0.034 for hs-cTnI). Conclusions: The biomarker-based ABC-stroke score was well calibrated and consistently performed better than both the CHA ₂ DS ₂ VASc and ATRIA stroke scores. The ABC score should be considered an improved decision support tool in the care of patients with atrial fibrillation. Clinical Trial Registration: URL: http://www.clinicaltrials.gov . Unique identifiers: ARISTOTLE, NCT00412984; RE-LY, NCT00262600.