The symptoms of Kawasaki syndrome (KS) suggest a possible relationship between KS and superantigen(s). The infrequent occurrence of KS among young infants may be due to a passive maternal antibody. We investigated the antibody titers for superantigens (toxic shock syndrome toxin [TSST]-1, staphylococcal exotoxin B, and streptococcal pyrogenic exotoxins C and A) in 15 patients with KS who were <6 months of age prior to gamma globulin therapy and in 10 mothers of patients with KS <6 months of age. Significant findings were observed for only TSST-1 among the 4 anti-superantigens. The proportion of patients with KS who had high anti-TSST-1 titers was significantly higher than that among infant control subjects (33% vs. 5%, respectively; P=.031). The mean anti-TSST-1 titer for the mothers was significantly lower than that of adult control subjects (P=.021). Among infants <6 months of age, TSST-1 may be related to KS, and a maternal antibody may protect infants from developing KS.