A patient with hyperthyroidism, heart failure, and depressed ejection fraction, in whom myocardial damage was evidenced by In-111-labelled monoclonal antimyosin antibodies, is presented. Both myocardial damage and left ventricular dysfunction disappeared after antithyroid therapy. This suggests that, in addition to volume overload, heart failure in hyperthyroidism could be also due to the presence of potentially reversible concurrent myocardial damage.