Little is known about the platelet dynamics and the effect of antiplatelet therapy in Kawasaki disease (KD). The aim of this study was to clarify platelet activation dynamics in acute-phase KD patients by assaying platelet-derived microparticles (PDMPs). The PDMP level in 18 patients with acute KD was measured on ELISA. Of the 18 patients, 14 were receiving oral aspirin and iv immunoglobulin (IVIG) and 4, oral aspirin alone. Blood samples were drawn before, immediately after, and 10–14 days after IVIG infusion; thereafter, at 1, 2, and 3 months after the onset of disease. PDMP level before aspirin treatment was significantly higher in acute-phase KD patients than in the control subjects with common febrile diseases (P< 0.01). In the acute-phase KD patients, IVIG significantly decreased PDMP level; the PDMP level was not lower on the similar day of KD in the patients who did not receive IVIG. Eight patients’ PDMP level rebounded after aspirin was discontinued. Platelets are activated during acute-phase KD, which confirms the importance of antiplatelet therapy. In addition, platelet activation continues as long as 2 or 3 months after the acute phase, the time at which aspirin is commonly discontinued, and the timing of aspirin discontinuation should therefore be evaluated in each individual patient.(Circ J 2014; 78: 188–193)