A male infant, aged 2 month, with Kawasaki disease had a myocardial infarction despite intravenous infusions of gamma globulin and aspirin at high dosage. He developed progressively a thin walled, dilated aneurysm of the apex of the left ventricle which became lined with thrombus despite treatment with warfarin. Another boy, aged 6 years, was noted on the 10th day of the evolution of Kawasaki disease to have developed a giant aneurysm of the main stem of the left coronary artery. Despite infusion of gamma globulin, the aneurysm remained unaltered and developed a thrombus. The thrombus resolved following treatment with warfarin, though the giant aneurysm has persisted. These two cases illustrate the serious consequences that can follow Kawasaki disease despite management optimal by current standards.