In order to elucidate the incidence of undesirable effects of amiodarone on thyroid activity, the authors examined a group of 11 patients who used this preparation for 8 to 44 months on account of disorders of the cardiac rhythm. In two patients clinical examination revealed obvious thyrotoxicosis, which after discontinuation of amiodarone receded spontaneously in the first patient in the course of five months, in the second patient within two months. In the third patient in this group the result of the thyreoliberin test (TRH) suggested subclinical hypothyroidism. In the remaining eight patients with clinical euthyroidism in none all results of laboratory examinations (basal thyroxine values, serum triiodothyronine, thyrotropin response to TRH administration) were in the range of reference values. Amiodarone-induced dysfunction of the thyroid gland--not only dysfunction detectable by laboratory examination, but also clinically apparent, thus is not a rare phenomenon. It is therefore indicated that amiodarone administration should be preceded and associated with examinations focused on thyroid function.