Sera of 42 patients with Kawasaki syndrome were evaluated for the presence of circulating immune complexes (CICs) by Raji cell radioimmune and Clq solid phase assays. Overall 69% of sera tested were positive by one or both assays, 52% by Raji cell radioimmune and 48% by Clq solid phase assay. CICs were detected most frequently 2 to 4 weeks after the onset of illness, but their presence did not correlate with the clinical severity of the illness or incidence of coronary artery aneurysms. Antibody to mite antigen was found in only 2 of 30 patients tested, and only one of these had CICs. CICs are frequently associated with Kawasaki syndrome, but their role in its pathogenesis remains to be determined.