The effect of thyroid disease on the heart, and especially on the electrocardiogram, has been followed in the present investigation chiefly from two points of view. In twenty-three cases we have compared the severity of the disease, as shown by the variation of the total metabolism from the normal, with the amplitude of the T-wave in the electrocardiogram. In the second place, in forty-seven cases of hyperthyroidism we have determined the frequency and type of arrhythmia. Also heart rates, blood pressures and electrocardiographic evidence of left ventricular preponderance and of general muscular tremor have been of interest. The basal metabolism was determined by means of the Benedict Unit apparatus.1 The electrocardiograms were taken with the Cambridge Model of the Einthoven string galvanometer and carefully standardized. It has been said that the T-wave is high in hyperthyroidism and low, absent, or even inverted in hypothyroidism. In our series of cases there