Purpose: Exaggerated pro-inflammatory reactions during the acute phase of Kawasaki disease (KD) suggest the role of immune dysregulation in the pathogenesis of KD. We investigated the profiles of T regulatory cells and their correlation with the clinical course of KD. Methods: Peripheral blood mononuclear cells were collected from 17 KD patients during acute febrile and subacute afebrile phases. T cells expressing CD4, CD25, and Foxp3 were analyzed using flow cytometry, and the results were correlated with the clinical course of KD. Results: The percentage of circulating $CD4^+CD25^{high}Foxp3^+$ T cells among $CD4^+$ T cells was Significantly higher during the subacute afebrile phase than during the acute febrile phase ($1.10%{pm}1.22%$ vs. $0.55%{pm}0.53%$, P=0.049). Although levels of $CD4^+CD25^{low}Foxp3^+$ T cells and $CD4^+CD25^-Foxp3^+$ T cells were only slightly altered, the percentage of $CD4^+CD25^+Foxp3^-$ T cells among $CD4^+$ T cells was significantly lower during the subacute afebrile phase than during the acute febrile phase ($2.96%{pm}1.95%$ vs. $5.64%{pm}5.69%$, P=0.036). Consequently, the ratio of $CD25^{high}Foxp3^+$ T cells to $CD25^+Foxp3^-$ T cells was higher during the subacute afebrile phase than during the acute febrile phase ($0.45%{pm}0.57%$ vs. $0.13%{pm}0.13%$, P=0.038). Conclusion: Decreased $CD4^+CD25^{high}Foxp3^+$ T cells and/or an imbalanced ratio of $CD4^+CD25^{high}Foxp3^+$ T cells to $CD4^+CD25^+Foxp3^-$ T cells might playa role in KD development. Considering that all KD patients were treated with intravenous immunoglobulin (IVIG), recovery of $CD4^+CD25^{high}Foxp3^+$ T cells during the subacute afebrile phase could be a mechanism of IVIG.