Background: Oxidative stress (OS) contributes to the acute phase of Kawasaki disease (KD) in a manner that is as yet unknown. In the present study OS in the acute phase of KD was investigated by measuring urinary 8-iso-prostaglandin F2α (8-iso-PG) and evaluating its correlation to the efficacy of intravenous immunoglobulin (IVIG) administration. Methods and Results: The 62 patients with acute phase of KD were enrolled, as well as 20 healthy children (HC) and 20 with acute febrile illness (FI). Urinary samples were obtained before and after administration of IVIG. The HC and FI groups also had inflammatory markers evaluated at the same time. The 8-iso-PG was significantly elevated in the 62 KD patients (719 ±335 pg/mg Cr) without IVIG administration compared with those with FI (583 ±213 pg/mg Cr) as well as HC (443 ±288 pg/mg Cr) (P<0.01). 40 patients were given 3 different regimens of IVIG: 16 received 2 g/kg for 1 day; 17 received 1 g/kg for 1 day; 7 received 400 mg · kg^-1 · day^-1 for 5 days. All regimens of IVIG reduced the 8-iso-PG level at 7 days after initiation. Conclusions: OS provokes vasculitis in KD, the activation of which was reduced by IVIG. The urinary level of 8-iso-PG is a useful marker of the effectiveness of IVIG in the acute phase of KD. (Circ J 2009; 73: 1315-1318)