Sixty patients with Kawasaki disease in the acute phase were studied using a two-dimensional echocardiography. Coronary lesions in Kawasaki disease were characterized echocardiographically by an increase of echo density of the coronary artery and by dilatation or aneurysm of the coronary artery. These findings appeared in the sequence mentioned above in patients who were studied serially. In the group treated with corticosteroid, 62% of the patients showed a disappearance or an improvement of the coronary dilatation or aneurysm, while only 33% of untreated patients showed such an improvement. In patients with coronary aneurysms on the initial echocardiogram, 53% of the pulse therapy group showed an improvement, but none of the untreated group did. A similar result was obtained by an analysis of its therapeutical effect, according to the number of vessels involved. Thus, corticosteroid pulse therapy was shown to be effective for the prevention of coronary arterial aneurysms in Kawasaki disease, if it was given in a sufficient dose in the acute phase of this disease.