We examined data from a large cohort of subjects with atrial fibrillation to determine whether men and women had different risks for atrial fibrillation-related thromboembolism. Women with atrial fibrillation who were not taking warfarin had a 60% higher rate of stroke than did men, independent of other clinical risk factors for stroke. When taking warfarin, both women and men had significant reductions in stroke risk without differences by sex in the rates of bleeding complications. These findings support the addition of female sex to traditional stroke risk stratification schemes for assisting in antithrombotic decision making for patients with atrial fibrillation.