<jats:p><jats:bold>Thromboembolic Risk Stratification in AF. <jats:italic>Background</jats:italic></jats:bold>: Antithrombotic recommendations for relatively low risk patients with atrial fibrillation (AF) are not well established. Some patients with CHADS<jats:sub>2</jats:sub> score = 0 have a CHA<jats:sub>2</jats:sub>DS<jats:sub>2</jats:sub>–VASc score of 2, which indicated warfarin therapy in the latter system. We evaluated the thromboembolic risk in AF patients with a CHADS<jats:sub>2</jats:sub> score of 0 or 1.</jats:p><jats:p><jats:italic>Methods:</jats:italic> A total of 695 patients with AF that were followed for ≥ 12 months (median 65.6 months, range 12–138 months), were analyzed retrospectively. The modified CHADS<jats:sub>2</jats:sub> score (MCS) was applied as follows. Each CHADS<jats:sub>2</jats:sub> score group was divided into 2 groups, A and B (i.e., MCS 0A vs 0B, and MCS 1A vs 1B) according to the number of nonmajor risk factors (female gender, chronic kidney disease, coronary artery disease, age 65–74 years). Group A had 0 or 1, and group B had 2 or more nonmajor risk factors.</jats:p><jats:p><jats:italic>Results</jats:italic>: In patients with CHADS<jats:sub>2</jats:sub> score = 1, there were 13 thromboembolic events (0.65%/year) in 343 MCS 1A patients, and 12 thromboembolic events (1.90%/year) in 108 MCS 1B patients. Thromboembolic risk was significantly higher in the MCS 1B compared to the MCS 1A patients (P = 0.006). In 244 patients with CHADS<jats:sub>2</jats:sub> score = 0, the thromboembolic risk of MCS 0B was similar to that of MCS 0A (P = 0.095), and 26 patients had a CHA<jats:sub>2</jats:sub>DS<jats:sub>2</jats:sub>–VASc score of 2.</jats:p><jats:p><jats:italic>Conclusion</jats:italic>: Patients with MCS 1B had a higher thromboembolic risk than patients with MCS 1A. Antithrombotic strategies for patients with a CHA<jats:sub>2</jats:sub>DS<jats:sub>2</jats:sub>–VASc score of 2 but a CHADS<jats:sub>2</jats:sub> score of 0 need further investigation. <jats:italic>J Cardiovasc Electrophysiol, Vol. 23, pp. 155‐162, February 2012)</jats:italic></jats:p>