There is marked activation of the endothelium and immune system in Kawasaki disease. Anticardiolipin antibodies (aCL) can cause activation of the endothelium. We measured aCL levels in acute Kawasaki disease patients and compared them to other febrile patients to see whether their aCL responses were different. Twenty-one patients with acute Kawasaki disease and 16 patients with an acute febrile illness were recruited. The aCL levels were measured in the sera of febrile patients and in Kawasaki disease patients prior to immunoglobulin therapy. There was no significant difference between the IgG aCL levels (p = 0.87) between the Kawasaki disease and febrile patients. However, the IgM (p = 0.01) and IgA (p = 0.03) aCL were significantly higher in patients with acute Kawasaki disease than in febrile children. Elevation of IgA aCL has been reported in association with other vasculitides and IgA-secreting plasma cells have been demonstrated in the vascular tissue in Kawasaki disease.