The case records of 13 children with hyperthyroidism were reviewed and an evaluation of their electrocardiograms made. The findings indicated that atrial and left ventricular hypertrophy are much more common in children than in adults. A significant number of patients also showed minor nonspecific S-T and T changes and prolongation of the P-R interval. There are findings to suggest that intrinsic cardiac pathological changes may occur in some patients with thyrotoxicosis and that it may manifest after a period of time. It appears, however, that the electrocardiographic changes observed may be reversible at varying periods of time after an euthyroid state is attained. An increased awareness of the electrocardiographic abnormalities in hyperthyroid children is emphasized as they differ to some degree from those of the adults.