Changes in cardiac troponin I (cTnI) in Kawasaki disease before and after treatment with intravenous gamma-globulins (i.v.GG) were investigated in 42 cases in order to confirm the usefulness of cTnI as a serological test for the early onset of acute myocarditis and the effectiveness of i.v.GG as treatment for it. The level of cTnI before i.v.GG treatment was increased in 42.9% (18/42), but 89% (16/18) were in the normal range after treatment as shown by improved clinical symptoms and signs; 9.5% (4/42) were treated twice with i.v.GG because of recurrent high fever, with a positive level of cTnI in the second test. Despite the improved clinical symptoms and signs after retreatment with i.v.GG, 4.7% (2/42) continued to have an increased level of cTnI in follow-up studies. In conclusion, the measurement of cTnI is a useful serologic test for the early diagnosis of myocarditis or myocardial cell injury and for confirming the effectiveness of i.v.GG therapy for the cure and prevention of cardiovascular abnormalities in KD patients.