This work was performed in order to evaluate the weight of hyperthyroidism on the genesis of atrial fibrillation in elderly subjects. The data are from the CASTEL (CArdiovascular STudy in the ELderly), an epidemiologic study performed in a town of northern Italy (Castelfranco Veneto), whose 3088 elderly subjects were called and 2254 enrolled for a 7-year intervention trial. From 2224 elderly persons examined in the present study, 90 had atrial fibrillation (AF) as determined by the presence of Minnesota Code 8-3; the other 2134 were used as control population. In the 90 with AF and in the randomly chosen controls, the thyroid function was studied by means of the TRH-test. Taking into consideration an increase of TSH greater than 0.5 or greater than or greater than 1 muUI/ml over the basal value after TRH administration, 5.5% of subjects with atrial fibrillation had a suppressed response (i.e. hyperthyroidism); taking into consideration a peak value of TSH greater than or equal to 2.3 muUI/ml irrespective to the basal value, the prevalence of hyperthyroidism was higher (17.8%), but not different than in control subjects. In conclusion, hyperthyroidism is frequent in elderly subjects but it does not play a role in the pathophysiology of AF. On the contrary, AF may be explained in the majority of cases by concomitant cardiovascular disease, i.e. left atrial enlargement, arterial hypertension, myocardial ischemia, and heart failure.