In 28 patients with Kawasaki disease, the relation of specific echocardiographic findings identified during the acute study of the illness, including valvular regurgitation, to development of coronary aneurysms was evaluated. Initial studies were performed at the time of clinical presentation, 5 to 10 days after the onset of fever, and follow-up studies were performed 1 to 2 months later. Patients in whom coronary aneurysms developed were more likely to have pericardial effusion (p = 0.0006) or mitral regurgitation (MR) (p = 0.014) at initial echocardiographic study than those without aneurysms. Presence of either mitral regurgitation (MR) or pericardial effusion had a positive predictive value of 0.84 for aneurysm development. Twenty-three percent of patients had MR, and it was associated with mild LV dilatation (35 +/- 3 vs 32 +/- 5 mm, p less than 0.05). Insufficiency of other valves was rare. Thus, MR and pericardial effusion on acute phase echocardiographic examination may predict development of coronary aneurysms in Kawasaki disease. Mild MR occurs frequently in acute Kawasaki disease and is associated with mild LV dilation.