Hyperthyroidism patients can develop atrial fibrillation (AF) with a frequency range between 10 and 25%. The Authors consider 256 hyperthyroid patients operated in their Department, from 1985 to 2001. Before and after surgery, thyroid examination, routine blood tests and common diagnostic evaluations were performed. The follow-up has been 12 months and during this period a L-Thyroxine (50-200 mg/die) therapy has been arranged. Before surgery 59 patients (23%) showed an AF associated with palpitations (76%), asthenia (66%), tremors (54%) and dyspnea (43%). The onset of AF was less than 1 months in 18% of cases, less than 3 months in 30% and chronic in remaining 52%. The 87% of patients was already pharmacologically treated. All the patients were treated with anticoagulants, with a INR value between 2 and 3. After the surgery, 47% of patients presented a restoration of the sinusal rhythm, while the 53% showed a stronger therapeutic responsiveness to specific antiarrhythmic therapy. Treatment of FA hyperthyroid patients with FA tends to normalize the function of the thyroid gland (euthyroidism). In our experience the restoring of sinusal rhythm usually develops in patients with 50 years of age (45 +/- 5 years), while in older patients AF tends to be resistant to treatment.