Interleukin 6 (IL-6) has manifold biological functions involved in the immune or inflammatory responses of the host to various stimuli. Here we asked whether IL-6 might be responsible for manifestations of Kawasaki disease (KD), such as immunoglobulin hypersecretion, lymphocyte activation and systemic vasculitis. IL-6 activity in the serum was determined by a sensitive colorimetric assay using an IL-6-dependent murine hybridoma clone. Usually sera from healthy or afebrile donors contained only negligible levels of IL-6 activity below the detection threshold of the assay. Importantly it was found that serum IL-6 was markedly elevated in all patients with acute KD. Serum levels of IL-6 activity gradually diminished during the course of the disease and reached undetectable or lower levels at the convalescent phase. However, such elevated levels of serum IL-6 activity were also observed in the majority of other febrile diseases, such as bacterial or viral infections, indicating that the appearance of IL-6 in the serum could generally occur in febrile or inflammatory disease conditions. Serum IL-6 activity correlated with serum concentrations of some acute phase proteins (APP), such as C-reactive protein, haptoglobin and alpha 1-acid glycoprotein, implying its role for modulating induction of APP in vivo. IL-6 is well known to be secreted by a variety of cell types. Further studies, including immunohistochemical analysis using anti-IL-6 antibody, will be necessary to examine whether the source of serum IL-6 in KD might be different from that seen in other diseases.