Background and Purpose- The risk of cardioembolic stroke in patients with atrial fibrillation (AF) cannot be accurately assessed and novel tools are needed to improve prediction. We hypothesize that telomere shortening constitutes a novel risk factor for cardioembolic stroke in patients with AF. Methods- The peripheral blood leukocyte telomere length (LTL) was determined by real-time polymerase chain reaction in 187 patients with AF, 93 of them without stroke history and 94 of them having suffered 1 cardioembolic stroke. Percentiles were calculated according to LTL values in the nonstroke group to estimate the cardioembolic stroke risk associated with LTL using logistic regression models. Results- Short LTL values were independently and dose-dependently associated with an increased risk of cardioembolic stroke, with an odds ratio (95% confidence interval) of 2.93 (1.24-6.94) and 6.26 (2.01-19.52), respectively, for sex, hypertension, diabetes mellitus, heart failure, and age-adjusted models using the LTL 10th and 5th percentile cut-offs, respectively. Conclusions- Telomere shortening is associated with cardioembolic stroke risk in patients with AF. Prospective studies are encouraged to establish the value of LTL to improve prediction tools to categorize cardioembolic stroke risk in AF.