The incidence of coronary obstruction subsequent to giant coronary aneurysm in Kawasaki disease was studied. In 20 cases, aneurysms with a maximal diameter greater than 8 mm were identified by coronary angiography 2 to 120 months (mean 16.9 months) after onset. There were 25 giant aneurysms among these 20 patients, all of whom underwent coronary angiography between 12 and 134 months (mean 31.7 months) after initial examination. Coronary obstruction occurred in six cases (30.0%), all within 4 years of onset of disease. There were five obstructive aneurysms in the right coronary artery (5/12; 41.7%) and two in the left coronary artery (2/13; 15.4%). One of these patients developed symptomatic myocardial infarction. Two had abnormal electrocardiographic findings suggesting myocardial infarction. In five cases, persistent perfusion defects were found by myocardial imaging. On the other hand, in two patients giant aneurysms persisted without obstructive changes for greater than 10 years. In both cases the aneurysm was in the left coronary artery and obstruction of the right coronary artery was found at initial angiography. These results seem to indicate the limited efficacy of antiplatelet therapy for giant coronary aneurysms in Kawasaki disease and that giant aneurysms are likely to progress to become obstructive within a few years even if antiplatelet therapy is given. Other forms of treatment, including surgery, should therefore be considered in such patients.