The cause of Kawasaki disease remains a mystery. Since its original description, many theories regarding the etiology of this serious childhood illness have been proposed, only to be refuted on closer scrutiny. The past year has seen important advances in our understanding of the pathophysiology of Kawasaki disease. Probably the most exciting advance has been the description of the mechanisms underlying the immunologic derangements seen in Kawasaki disease, together with evidence that this mysterious illness may be caused by a toxin that acts as a superantigen.