There are some cases of Kawasaki disease (KD) complicated with coronary aneurysm (CA) in spite of intravenous administration of gammaglobulin (IVGG). We prospectively tested whether plasma endothelin-1 (ET) could be a predictive factor of CA. This study consisted of 60 patients in acute phase of KD. There were 34 male subjects and 26 female subjects aged 4 months-9years. All patients were proved normal coronary arteries by echocardiography when hospitalized before treated with oral aspirin and IVGG therapy. 9 patients were complicated with CA revealed by echocardiography every 2nd or 3rd day. Blood samples for ET were obtained from veins during febrile period before 5th day in hospital and in afebrile period. ET was measured by radioimmunoassay by using ET antibody. Normal range of ET measured by this method is 4.0 pg/ml (0–1 year) or 2.0 pg/ml (2years or older). We evaluated corrected ET (ratio of measured ET to normal upper limit for age). In patients with intact coronary artery, corrected ET were 0.96 ± 0.47 (febrile period) and 0.59 ± 0.22 (afebrile period). In patients complicated with CA, corrected ET were 1.63 ± 0.50 (febrile; p < 0.05) and 1.45 ± 0.38 (afebrile; p < 0.01) and all of them with CA sustained higher ET level than normal range until afebrile period. Specificity of prediction for CA was 66.7%, sensitivity of it was 100% as result. We supposed that plasma ET is a good predictive factor of CA in KD.