Clinical and epidemiologic features suggest infectious agent or agents as a possible cause of Kawasaki disease; however, the etiology of Kawasaki disease still remains unknown. A number of microorganisms including bacteria, viruses, rickettsiae etc. were hypothesized as an etiology of the illness. Unfortunately, no specific agent that provides reproducible evidence has yet reported. The differences of histology between vasculitic disorders indicate the differences of etiology and pathogenesis. The histological characteristics that abnormal activation of monocytes/macrophages is caused in the vascular lesions of Kawasaki disease can be of help when considering the etiology of Kawasaki disease.