Kawasaki syndrome (KS), the main cause of acquired heart disease in children, is associated with the selective expansion of V beta 2+ T cells in peripheral blood. Our study suggests that KS may be caused by a superantigen--a staphylococcal or streptococcal toxin. Bacteria were cultured without knowledge of their origin, from the throat, rectum, axilla, and groin of 16 patients with untreated acute KS and 15 controls. Bacteria producing toxins were isolated from 13 of 16 KS patients but from only 1 of 15 controls (p < 0.0001). Toxic shock syndrome toxin (TSST) secreting Staphylococcus aureus was isolated from 11 of the 13 toxin-positive cultures, and streptococcal pyogenic exotoxin (SPE) B and C were found in the other 2. These toxins are known to stimulate V beta 2+ T cells. All TSST-producing KS isolates were tryptophan auxotrophs indicating they were clonally related. S aureus isolates from acute KS patients were unusual because they produced less lipase, haemolysin, and protease compared to other isolates (p < 0.01). S aureus colonies from KS patients were white, and could be easily mistaken for coagulase-negative staphylococci, whereas colonies of non-KS isolates were gold. These observations suggest that the expansion of V beta 2+ T cells in most patients with KS may be caused by a new clone of TSST-producing S aureus, and, in a minority of patients, SPEB-producing or SPEC-producing streptococci.