We evaluated whether there is a possible relationship between the effectiveness of gamma-globulin treatment for patients with Kawasaki disease (KD) and the polymorphism of Fcgamma RIIa, IIIb, and IIIa. Genomic DNA was extracted from whole blood collected from 56 patients with KD who received gamma-globulin treatment. The genotypes for Fcgamma RIIIb-NA(1, 2), Fcgamma RIIa-H/R131, and FcgammaRIIIa-F/V158 were determined to investigate the association between these polymorphisms and the development of coronary lesions (CALs). Twenty-three percent of patients with the HH allele for the Fcgamma RIIa polymorphism progressed to CALs, compared with 60% with the HR and RR alleles. HR and RR alleles may be a predictor of the progression of CALs in KD before the initiation of gamma-globulin therapy.