Kawasaki disease (KD) is an acute febrile vasculitic syndrome with thrombocytosis occurring in childhood. Transient thrombocytosis of KD is sometimes the cause of complications such as coronary aneurysmal thrombosis and myocardial infarction. We have analysed blood TPO levels in KD and found that 26/31 acute-phase KD patients had detectable blood TPO levels (mean 173 pg/ml; range 89-294 pg/ml), which decreased immediately with the elevation of platelet counts in 5/12 patients studied. Elevated serum level of TPO may have an important role for this ill-defined disease.