Myocardial failure may complicate hyperthyroidism. Some authors consider that preexisting myocardial lesions are necessary for its development. We studied a case of myocardial failure, presenting as a dilated cardiomyopathy, complicating hyperthyroidism in a 57 year old woman. She had a bio-clinical evaluation and a haemodynamic study with endomyocardial biopsy of the left ventricule. No valvular or coronary disease were noted. The light and ultra-microscopic aspects of the myocardium were within normal limits. We conclude that preexisting myocardial lesions are not essential to the development of myocardial failure complicating hyperthyroidism.