Review of 523 confirmed cases of Kawasaki syndrome reported to the Centers for Disease Control (CDC), during the period from July 1976 through December 1980, disclosed a previously unrecognized seasonal variation, with a peak incidence between February and May. A significantly higher incidence in black children, compared with white children, was also noted. Earlier reports of a higher incidence in children younger than 5 years of age, males, and children of Asian ancestry were confirmed. Frequently reported complications included joint involvement (27%) and cardiac abnormalities (22%); 1.2% of patients died. Four outbreaks (mean duration, 3.8 months) were investigated by the CDC, but no evidence of person-to-person transmission or a point source of exposure was found. These data suggest that some cases may be caused by an exogenous agent or toxin that is most prevalent in the late winter and spring. In addition, host, environmental, or other cofactors are likely to be important determinants of susceptibility.