Our objectives were to describe epidemiological pictures of infant Kawasaki disease (KD) and to report the youngest patient ever reported in Japan. A Japanese database of 105755 KD patients reported in the 25 year period since 1970 was analyzed. Of all the cases registered, there were only six cases aged 30 days or younger and 1768 cases (1.67%) aged 90 days or younger. We reported a typical case of KD (female, the onset at 20 days old) who was the youngest patient ever reported in Japan. The epidemiology of KD in early infants does not contradict the infection theory of the etiology; the rare incidence in the neonatal period can be explained by the protective effects of the passive immunity transferred from the mother and by exposure to the unknown infectious agents.