Intensive echocardiographic studies were undertaken in 37 patients with Kawasaki disease (KD) and 12 febrile controls during the acute stages. Video-tape recordings were read doubly-blinded by 3 pediatric cardiologists at 2 hospitals. Three characteristics of subtle coronary arterial lesions, including contour of lumen, smoothness of endothelium and sono-homogeneity of the surrounding tissue were particularly described. Seven of the 37 KD patients (19%) were found to have coronary arterial aneurysms (CAA) at the end of the convalescent stage. The 3 cardiologists agreed in the diagnosis of the subtle lesions by 72%. More patients with Kawasaki disease were found to have an irregular endothelium than the controls (P < 0.001). KD patients later complicated with CAA were more likely to have had the three characteristics of subtle coronary lesions during the acute stage than the KD patients without aneurysms (P < 0.001). Also, the coronary arterial size measured during the acute stage were found to be positively correlated with their largest dilatation at the convalescent stage (P = 0.0001, r = 0.61). We conclude that early subtle coronary arterial lesions can herald the development of coronary arterial aneurysms.