Transitory flattening or inversion of the T wave of the electrocardiogram has been found during the active stage of rheumatic fever by Rothschild, Sacks and Libman, by Shapiro, and by Master. However, Schwartz and Weiss have claimed that the T wave of children with acute rheumatic fever does not differ from the T wave of the normal child. In view of this difference of opinion, it was decided to reinvestigate this subject. Electrocardiograms of children suffering from rheumatic fever were selected for this study from the 8300 records in the electrocardiographic files of the Heart Station of Michael Reese Hospital. From these were selected those in which 2 or more electrocardiograms had been taken. The clinical charts of these patients were studied, and 12 cases, with electrocardiograms in both the active and inactive periods of the disease, were selected. The rheumatic fever was assumed to be active when a majority of the following signs and symptoms was found: rapid heart rate, pyrexia, “sore throat”, joint pains, acute pericarditis and an elevated white blood count. The electrocardiographic changes found in these cases confirm the observations of Cohn and Swift, Rothschild, Sacks and Lib-man, Shapiro, and Master,i. e., the P wave was often altered in size, shape and direction; the P-R interval was often lengthened; the QRS complex was often notched, lengthened, or changed in amplitude or direction; the S-T segment was often anisoelectric or sloping; and the T wave was flattened, diphasic, isoelectric or inverted. With recovery from the active stage of the disease there was a tendency, after a varying period of time, for the electrocardiogram to return towards normal. Typical changes are shown in Figs. 1 and 2. The T wave during the active phase not only had become small but usually had lost its normal peaked top (Fig. 1, Record A). When the T wave was inverted, it tended to assume the appearance of the coronary T wave (Fig. 2, Record C, Lead III). With recovery from the active stage of rheumatic fever, the T wave tended to become taller and sharply peaked (Figs. 1 and 2. Records B and D). Some cases were followed through successive active episodes and intervening inactive periods, and the same changes were found with each new cycle of the disease (Fig. 2). T wave changes similar to those described in acute rheumatic fever, but more transitory, have been described in pneumonia (Master, Romanoff and Jaffe; Arnett and Harris; Abt and Vinnecour), in typhoid fever (Chagas), and in scarlet fever (Shookhoff and Taran). Summary. A study was made of the electrocardiograms of 12 cases of rheumatic fever in children, taken during both the active and inactive stages of the disease. During the active stages, the T wave was. softly rounded and small, sometimes isoelectric, diphasic or inverted, and during recovery the T wave became upright, tall, and sharply peaked.