To determine whether changes in ECGs correlated with abnormalities in echocardiograms in children with acute and convalescent Kawasaki syndrome, we undertook a retrospective analysis of 44 patients examined during the first 2 weeks of illness and followed for at least 6 months, 31 for longer than 1 year. We analyzed 360 ECGs, 282 echocardiograms, and clinical status. Results of echocardiography showed 18 children with myocardial dysfunction, pericardial effusion, and/or coronary arterial abnormalities (41%); 26 children had no abnormalities. All 18 of the former and all but 10 of the latter had abnormalities on serial ECGs that normalized on recovery. Thirty-four of 44 patients (77%) had ECG abnormalities. These abnormalities were most prevalent in the first month: 68% appeared in the first week, about 50% in the second to fourth weeks, 16% at 2 months, and 10% at 3 months. The type of ECG change did not predict the type of echocardiographic abnormality, except that low voltage of QRS was found only with pericardial effusion. The most frequent early changes were in T waves, which were flattened in 28 (64%). Later 12 patients had high, peaked, abnormal T waves. Prolonged PR interval occurred in 13 patients, seven of whom had echocardiographic abnormalities. Of six with prolonged QT interval, four had abnormal echocardiograms. The possibility of having an abnormal echocardiogram increased with the number of ECG changes: from 0 risk with no change to 37% for one, 47% for two, 80% for three, and 100% for four changes in serial ECGs. Both ECGs and echocardiograms are important in early and subsequent evaluation of these patients. Inasmuch as the ECGs showed abnormalities in 77% whereas echocardiograms showed changes in 41%, we believe that serial ECGs in comparison with prior tracings offer the more sensitive noninvasive indicator of pancarditis in young children during acute and convalescent Kawasaki syndrome.