Kawasaki disease is a form of acute febrile multisystem vasculitis. In Japan, the Kobayashi score is used to predict intravenous immunoglobulin resistance and subsequent complications, including coronary artery abnormalities, in children with Kawasaki disease. If the score is high, it is standard practice to use steroids in the initial treatment of the disease. In this study, we describe a pediatric case of coronary artery abnormality formation after acute steroid treatment for Kawasaki disease, despite the lack of an inflammatory response and Kawasaki disease symptoms. The use of steroids with acute Kawasaki disease patients can mask laboratory findings and clinical symptoms. Echocardiogram should be considered an equivalent technique to physical examination.