The aim of this study is to clarify the specificities of atrial fibrillation in hyperthyroidism. It's a retrospective study of 6 years, about 14 patients hospitalized for hyperthyroidism with atrial fibrillation. There were 9 Women and 5 men, 55.7 +/- 11.5 years old. Arryhthmia was discovered especially with palpitation and dyspnea. Cardiac echography diagnosed valvular disease in 83.3% of cases. Cardiovascular complications concerned 5 patients and consisted in cardiac insufficiency and cardiomyopathy in one case. Treatment of hyperthyroid consisted in radio-iodine administered to 7 patients at the dose of 9.8 +/- 3.9 mCi. Two patients had total thyroidectomy. Arryhthmia was treated with propranolol, 98.3 +/- 70 mg daily and anticoagulant treatment was given. From the group of nine hyperthyroid recovered patients, arryhthmia was reduced in 3 cases. In this group, age was higher and arryhthmia history was longer. We conclude that chances to treat arryhthmia associated to hyperthyroid are higher when euthyroidism or even hypothyroidism is rapidly obtained, during of atrial evolution is short and some factors aren't present, like aging, existence of valvular disease or left auricular dilatation.