The incidence of atrial fibrillation (AF) in 142 patients with thryotoxicosis was 21% and that of thromboembolic disease was 8.5%. Male sex, increasing age, and associated rheumatic or hypertensive heart disease were significantly associated with the presence of AF. Thromboembolic episodes occurred in 12 (40%) of the patients with AF and in none of the 112 patients without AF. Cerebral emboli accounted for 53% of the episodes. In view of the grave prognosis of the embolic episodes, patients with thyrotoxicosis should be treated with heparin during attacks of AF and with oral anticoagulants when they have chronic fibrillation.