A series of 51 cases of thyrotoxicosis gathered during a period of 6 years in a department with a cardiological orientation is analysed. This peculiar form of thyrotoxicosis is often unrecognized in view of its frequent occurence in aged subjects (which is misdiagnosed as "senile heart") and of the incidence in its causality of a toxic adenoma, the endocrine semiology of which is often reduced. In front of any case of heart failure of unknown cause, one should systematically think of a thyroid aetiology. A persistent sinus rhythm is not enough to discard it. A low cholesterol rate should induce to continue the investigation. The subnormal character of the classical laboratory parameters (including the thyroid hormone assay) does not always make it possible to exclude definitely the diagnosis. When the clinical picture is suggestive, only the radio-isotope examination with dynamic tests is liable to provide decisive data.