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) beta-chain usage of peripheral T cells. We detected significantly more oligoclonal T cells (15.5 plus/minus 8.9 bands representative for clonal T-cell expansions) in peripheral blood from ITP patients than from healthy donors (2.8 plus/minus 2.6 bands). Frequently used V beta genes in these accumulated T cells in ITP were V beta 3, 6, 10, 13.1 and 14. To determine whether these bands were derived from clonal T cells, presumbly in a preactivated state, we established some T-cell clones (expressing CD4 and TCR V beta 6, 13.1 or 14) by nonspecific stimulation from patients' peripheral mononuclear cells, and examined their clono-types. Clonal identities for three out of seven clones tested were confirmed using SSCP analyses to compare the migration of their beta-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.