Background Cardiac biomarkers are independent risk markers in atrial fibrillation, and the novel biomarker–based ABC stroke score (age, biomarkers, and clinical history of prior stroke) was recently shown to improve the prediction of stroke risk in patients with atrial fibrillation. Our aim was to investigate the short‐term variability of the cardiac biomarkers and evaluate whether the ABC stroke risk score provides a stable short‐term risk estimate. Methods and Results According to the study protocol, samples were obtained at entry and also at 2 months in 4796 patients with atrial fibrillation followed for a median of 1.8 years in the ARISTOTLE (Apixaban for Reduction in Stroke and Other Thromboembolic Events in Atrial Fibrillation) trial. Cardiac troponin I, cardiac troponin T, and N‐terminal pro‐B‐type natriuretic peptide were measured with high‐sensitivity immunoassays. Associations with outcomes were evaluated by Cox regression. C indices and calibration plots were used to evaluate the ABC stroke score at 2 months. The average changes in biomarker levels during 2 months were small (median change cardiac troponin T +2.8%, troponin I +2.0%, and N‐terminal pro‐B‐type natriuretic peptide +13.5%) and within‐subject correlation was high (all ≥0.82). Repeated measurement of cardiac biomarkers provided some incremental prognostic value for mortality but not for stroke when combined with clinical risk factors and baseline levels of the biomarkers. Based on 8702 person‐years of follow‐up and 96 stroke/systemic embolic events, the ABC stroke score at 2 months achieved a similar C index of 0.70 (95% CI, 0.65–0.76) as compared with 0.70 (95% CI, 0.65–0.75) at baseline. The ABC stroke score remained well calibrated using predefined risk classes. Conclusions In patients with stable atrial fibrillation, the variability of the cardiac biomarkers and the biomarker‐based ABC stroke score during 2 months are small. The prognostic information by the ABC stroke score remains consistent and well calibrated with similar good predictive performance if patients are retested after 2 months. Clinical Trial Registration URL: http://www.clinicaltrials.gov . Unique identifier: NCT 00412984.