The study was performed to investigate the level of serum cardiac troponin I (cTnI), a marker specific for myocardial damage, using a chemiluminescent immunoassay in the acute febrile stage of Kawasaki disease (KD). The study population consisted of 45 KD patients before intravenous gamma-globulin (IVGG) therapy and a control group of 20 patients without KD. Among KD patients the results from measurements of the level of cTnI were positive in 18 cases (40%) and the creatine kinase (CK)-MB was positive in 11 cases (24%), but in the control group both the cTnI and CK-MB results were negative. Seven KD patients (15.6%) showed increases in both cTnI and CK-MB that were significantly correlated with each other (p < 0.05); however, CK-MB is not heart-specific. A significant increase in the level of cTnI in the acute stage of KD suggests that acute myocarditis or myocardial cell injury begins in the early phase of the disease (p < 0.05). The serologic test for cTnI can thus be a useful method for the early diagnosis of acute myocarditis and may enable early treatment with IVGG to reduce the cardiovascular abnormalities in KD patients.