Kawasaki disease (KD) is an acute systemic inflammatory illness of childhood that can result in coronary artery aneurysms, myocardial infarction, and sudden death. Clinical and epidemiologic data point to an unknown infectious agent as the cause. We discovered that an oligoclonal IgA immune response is present in arterial tissue in acute KD. Synthetic versions of prevalent IgA antibodies in the KD arterial wall identify cytoplasmic inclusion bodies in acute KD ciliated bronchial epithelium and other inflamed KD tissues. Light and electron microscopic studies show that the inclusion bodies are consistent with aggregates of viral protein and RNA, and are likely formed by the KD etiologic agent. KD susceptibility is likely to be polygenic. Treatment of gammaglobulin nonresponders usually consists of additional intravenous immunoglobulin, methylprednisolone, and/or infliximab. Additional data regarding KD pathogenesis are urgently needed to provide other targets for therapy for those patients at highest risk of developing coronary artery abnormalities.