Eighteen of a total of 22 serum specimens (81.8%) from patients with Kawasaki disease were positive for immunoglobulin G or M antibodies to human herpesvirus 6, whereas 10 of 16 age- and sex-matched healthy controls (62.5%) were seropositive. Additionally, increased geometric mean antibody titers of immunoglobulin G were shown in these patients. These results suggest that the status of human herpesvirus 6 infection may be a reflection of the immunologic alterations that are associated with Kawasaki disease.