The main cause of death in Kawasaki disease is myocardial infarction due to thrombotic occlusion of a coronary aneurysm. Intracoronary thrombolytic treatment was performed in 15 patients with Kawasaki disease with giant coronary aneurysms. Three patients had acute myocardial infarction, four demonstrated silent myocardial infarction, three suffered chest pain and five did not show ischemia features but had massive thrombus in the coronary aneurysms. Urokinase was infused into the coronary aneurysms as a bolus of 8,000 to 10,000 units/kg via a catheter over 10 minutes. Partial but significant coronary recanalization was achieved after injection of urokinase in a patient with acute myocardial infarction. Complete resolution of massive intracoronary thrombi was observed in 3 of 15 patents, and partial resolution was recognized in 4 cases. In 7 patients, the size of thrombus did not change. Recurrence of the thrombus was observed in 4 patients by serial two-dimensional echocardiography. Urokinase was readministered and two showed significant reduction in the thrombus. All patients have been followed for more than 2 years with longest 8 years (mean: 3.3 yrs), and none have had a recurrence of myocardial infarction or died. These findings suggest that intracoronary urokinase is useful for the treatment and prevention of myocardial infarction in Kawasaki disease.