Kawasaki disease (KD) is an acute febrile illness of infancy and early childhood. In spite of extensive studies, the cause of KD is not known. Interleukin 6 (IL-6) has manyfold biological functions involved in the immune or inflammatory responses of the host to various stimuli. Here the author investigated whether IL-6 might be responsible for manifestations of KD, such as immunoglobulin hypersecretion, lymphocyte activation and systemic vasculitis. Serum IL-6 levels in KD were determined by ELISA. Usually sera from healthy children contained only negligible levels of IL-6. Serum IL-6 was markedly elevated in all patients with acute KD, which gradually decreased during the course of the disease. Serum IL-6 correlated with serum concentration of C-reactive protein and with serum soluble interleukin-2 receptor level, but did not show any correlation with peak platelet count during subacute phase of the disease. Increased serum IL-6 level did not show any relation to development of coronary aneurysms and dilatation. Further studies will be needed to examine the source and the pathogenetic roles of increased serum IL-6 in KD.