Accurate evaluation of the extent of coronary artery lesions complicating Kawasaki disease is clinically important in patient management. Based on a total of 188 coronary angiograms and retrospective follow-up observations, the condition of coronary aneurysms was quantitatively graded as: 0 (normal)--no significant enlargement in any portion of the coronary artery; I (mild)--aneurysmal dilatation of the coronary artery evident but localized, with a maximal diameter of less than 4.0 mm; II (moderate)--maximal diameter of coronary aneurysms between 4.0 and 8.0 mm, regardless of body size; III (severe)--giant aneurysms, with the maximal diameter greater than 8.0 mm. Most mild coronary aneurysms regressed to normal within a short time, and the patient's prognosis was good. The course of grade II aneurysms varied, depending on initial angiographic coronary diameter, but all were eventually reduced in coronary size. In contrast, grade III aneurysms usually progressed to become obstructive or stenotic coronary lesions, or the large aneurysm persisted. Follow-up observations revealed that the course of coronary artery disease depended on the size and distribution of aneurysms at initial angiography. This grading of the severity of coronary lesions may provide useful criteria for predicting the prognosis of patients with Kawasaki disease.