The authors report on 30 cases of cardiothyrotoxicosis in the young adult with Basedow's disease. Cardiac arrhythmia represented the most frequent clinical form of dysfunction, mainly atrial fibrillation. Conductive disorders came second, with seven cases of first-degree atrioventricular block [AVB], one case of second-degree AVB, and two cases of sinusoidal bradycardia. Three cases of ballooning of the mitral valve were detected by echocardiography. Myocardial hypertrophy was found in one case. The authors discuss the various physiopathological hypotheses regarding conduction and myocardial hypertrophy anomalies. No cases of severe cardiac insufficiency or coronaropathy were noted, which is explained by the absence of cardiopathic antecedents and the young age of the patient population. Treatment is more complicated in the case of a preexisting cardiac event.