The rate of second-case Kawasaki disease occurring among 1788 siblings of children with the disease was derived from data obtained from questionnaires mailed to the members of the Japanese Association of Parents of Children With Kawasaki Disease. Within 1 year after the onset of the first case in a family, the overall second-case rate was 2.1% for siblings, as compared to an overall incidence of approximately 0.19% in the general population of children 0 to 4 years of age in Japan in the epidemic year 1982. For siblings younger than 1 year of age, it was 8.4%, and for those between 1 and 2 years of age, it was 9.3%. More than half (54.1%) of the second cases developed 10 days or less after the first cases occurred.