To determine whether clonal T cells accumulate in idiopathic thrombocytopenic purpura (ITP), we performed single-strand conformation polymorphism (SSCP) analysis to detect T-cell receptor (TCR) β-chain usage of peripheral T cells. We detected significantly more oligoclonal T cells (15.5 ± 8.9 bands representative for clonal T-cell expansions) in peripheral blood from ITP patients than from healthy donors (2.8 ± 2.6 bands). Frequently used Vβ genes in these accumulated T cells in ITP were Vβ 3, 6, 10, 13.1 and 14. To determine whether these bands were derived from clonal T cells, presumably in a preactivated state, we established some T-cell clones (expressing CD4 and TCR Vβ 6, 13.1, or 14) by nonspecific stimulation from patients’ peripheral mononuclear cells, and examined their clonotypes. Clonal identities for three out of seven clones tested were confirmed using SSCP analyses to compare the migration of their β-chain complementarity determining region 3 (CDR3) cDNAs, expanded by polymerase chain reaction (PCR) with those from peripheral blood. Therefore, distinctive T-cell clones accumulated in the periphery in ITP and they may be related to the autoimmune-mediated destruction of platelets.