Myocarditis is a well-recognized component of Kawasaki disease, with left ventricular dysfunction occurring in more than half of the patients during the acute phase of the disease. The purpose of our study was to evaluate myocardial function in these patients using the myocardial performance index, also known as the Tei index. In a prospective study, 14 patients underwent echocardiographic evaluation, first at the time of diagnosis of the disease, in its acute phase before treatment with intravenous immunoglobulin and then 2 weeks later after treatment with immunoglobulin. We assessed the Tei-index, the ejection fraction, shortening fraction, and the presence of valvar regurgitation, pericardial effusion, or coronary arterial involvement. As a control, we also assessed 22 healthy children, matched for age and sex with the study population. Of the patients, half had an increased left ventricular Tei-index in the acute phase, as compared with the data obtained after treatment, the index changing from 0.43 ± 0.08 to 0.35 ± 0.06 (p equal to 0.003). An increased index for the right ventricle was found in 5 patients (36%), values being 0.30 ± 0.05 as opposed to 0.26 ± 0.04 (p equal to 0.009). Of the patients, 5 (35.7%) also had decreased ejection fractions and proportional shortening fractions during the acute phase, confirming left ventricular dysfunction. We concluded that the Tei-index, which measures combined systolic and diastolic function, is a simple, sensitive, and accurate tool for estimating global myocardial dysfunction in patients with Kawasaki disease.