Two hundred one patients (138 boys, 63 girls), 1 month to 11 years old, with Kawasaki disease underwent coronary angiography, ultrasonic tomography, myocardial imaging, and biopsy of right ventricular myocardium. Aneurysms of epicardial coronary arteries, mostly left, occurred in 26 cases (12.9%). Degeneration and proliferation of endothelium, edema, scarring, and fibrosis, slight to moderate, involved the tunicae of intramural small vessels in all cases. These changes tended to abate with time. The basic lesion of Kawasaki disease, round-cell myocarditis and fibrosis, involved all cases and persisted. Disarrangement, abnormal branching, disarray, and hypertrophy of myocytes correlated significantly with myocarditis, but not with epimural or intramural coronary angitis. As a consequence of the myocardial changes, it is suggested that some cases may terminate as myocardiopathy. Scoring criteria for vascular and myocardial changes in biopsy specimens are included.