Hospital records of 229 children with diphtheria revealed evidence of myocarditis affecting 47 (20 per cent), both by physical signs and abnormal electrocardiograms. The 437 electrocardiograms taken on the 47 patients were analyzed by the conventional and the electrovectorcardiographic method of Grant, and the QRS-T spatial angles were evaluated. A high predictability of survival from diphtheria is afforded by the electrocardiogram. Cases were grouped according to their most severe ECG abnormalities. If only the P-R interval was prolonged (Group A), or the T vector was shifted (Group B), causing the QRS-T angle to widen, the prognosis for complete recovery was excellent. Prolongation of the P-R interval tended to persist, whereas primary T-wave changes subsided. An intraventricular block (Group C) involved the superior branch of the left main bundle branch to the left ventricle in 8 children; and of these, 4 died, but only one as a result of myocarditis. One child with only incomplete right bundle branch block recovered. Of 14 children with a major conduction block (Group D), 11 died. Six of these children had complete bundle branch block: there were 3 fatalities. Complete A-V block was the most ominous ECG sign, in that it was consistently followed by death in 8 children. However, in one of these children the block regressed and death resulted from polyneuritis and respiratory failure. Out of the total 229 cases, 15 deaths occurred in that group of 47 patients in whom there was evidence of myocardial involvement; 10 deaths were due to this involvement and peripheral vasodilatation. One child died early from obstructive, laryngeal diphtheria, and 4 succumbed to the late effects of progressive polyneuritis complicated by pulmonary hypoventilation and infection. Autopsy was carried out on 8 children, for whom correlations of ECG, clinical, and pathologic data were presented. Early death in 3 children (Cases 40, 44, and 47) was associated with complete A-V block and histologic evidence of extensive myocardial disease, and in one child (Case 35), with right bundle branch block and a hemorrhagic myocardium. The child who died early from laryngeal obstruction (Case 31) had parietal block by ECG and interstitial edema of the myocardium. Death due to peripheral neural and respiratory causes late in the course of illness of 3 children (Cases 33, 38, and 46) was associated with regression of ECG changes and much less severe histologic evidence of myocarditis.