To confirm the presence of antiendothelial cell antibodies (AECA) and study the clinical value of their measurement in Kawasaki disease (KD), sera from patients with KD were assessed for binding to human umbilieal vein endothelial cells (HUVEC) using a cellular based ELISA, and for complement-mediated cytotoxiety using in-labelled HUVEC. In the ELISA assay, IgM AECA were detected in 16/22 sera, whereas IgG AECA were only present in one out of 19 sera. There was a significant difference in IgM AECA titres when comparing patients with controls. Eight out of 16 sera showed cytotoxicity against HUVEC, and this was significantly enhanced when HUVEC were pretreated with tumour neerosis factor (TNF). IgM AECA titres measured by ELISA were positively correlated with their cytotoxieity. The findings suggest that IgM class AECA in sera from patients with KD are detectable by ELISA method, and that some of these antibodies. 50% in this study, can mediate complement-dependent cytotoxicity against endothelial cells.