Immunoregulatory CD4(+)CD25(+) T cells play an important role in the induction and maintenance of peripheral self-tolerance. These professional regulatory cells prevent the activation and proliferation of potentially autoreactive T cells that have escaped thymic deletion. Therefore, CD4(+)CD25(+) T cells are believed to possibly play an important role in pathogenic autoimmune diseases. We measured the count of CD4(+)CD25(+) T cells in 44 patients with idiopathic thrombocytopenic purpura (ITP), and the number of CD4(+)CD25(+) T cells and clinical features were then analyzed. By using a flow cytometric analysis, the number of CD4(+)CD25(+) T cells in the patients with ITP showed a very wide distribution in comparison to healthy volunteers. The number of CD4(+)CD25(+) T cells was significantly lower in the ITP patients in the severe phase, and in patients positive for anti-glycoprotein IIb-IIIa antibody. However, the number of those cells increased in the patients at the complete remission phase, especially after a splenectomy. The Foxp3 mRNA levels of peripheral blood mononuclear cells (PBMC) of ITP patients were higher with an improved platelet count than in those with a low platelet count. In addition, the Foxp3 mRNA levels closely correlated with the number of CD4(+)CD25(+) cells. These mechanisms remain to be fully elucidated, however, the count of CD4(+)CD25(+) T cells is considered to possibly be related to the severity of ITP.