Seventy-nine patients with mucocutaneous lymph node syndrome were evaluated prospectively by clinical examination, electrocardiography, chest radiography, M mode and two dimensional echocardiography and thallium-201 myocardial scanning. Serial changes were categorized according to the duration of illness: stage I (1 to 10 days), stage II (11 to 20 days), stage III (21 to 30 days), stage IV (31 to 60 days) and stage V (61 days to 40 months). The presence of myocarditis in stages I and II was suggested in 40 of 79 patients (50.6 percent) by electrocardiographic, echocardiographic, radiographic and clinical abnormalities. Myocarditis was accompanied by pericarditis in six patients and by both endocarditis and pericarditis in one patient. These signs of inflammation were resolved by stage III in all but three patients with electrocardiographic abnormalities. In the active stage, large coronary arterial lesions were suspected only because of an abnormal spherical echo-free space in the region of the coronary arteries on two dimensional echocardiograph as well as electrocardiographic evidence of deep Q waves in leads II, III and aVF. One or more coronary aneurysms developed in 11 patients, primarily in stage II; regression of the aneurysm was noted in 5 of these patients during stages III, IV and V. Aneurysm regression demonstrated by angiography did not correlate with echocardiographic changes in aneurysm size in one patient. Moreover, the occurrence of coronary aneurysm did not correlate with the presence of signs of carditis, because the frequency of carditis was the same in patients with and without aneurysm.