We identified all patients with an incident diagnosis of hyperthyroidism in the Danish National Patient Registry, and among those we identified subjects with a diagnosis of atrial fibrillation or flutter that occurred +/- 30 days from the date of the diagnosis of hyperthyroidism. Among 40,628 subjects with hyperthyroidism, 3,362 (8.3%) had a diagnosis of atrial fibrillation or flutter. Male sex, increasing age, ischemic heart disease, congestive heart failure and heart valve disease were associated with an increased risk of atrial fibrillation or flutter in patients with hyperthyroidism.