Eradication of Helicobacter pylori leads to platelet recovery in some patients with idiopathic thrombocytopenic purpura (ITP).Therefore, the pathogenesis of a subgroup of ITP is probably associated with H pylori infection (H pylori-related ITP). If H pylori-related ITP is a definite subgroup of ITP, specific oligoclonal T-cells might accumulate in the peripheral blood (PB). To address this issue, we performed single-strand conformation polymorphism analysis of complementarity-determining region 3 (CDR3) of the T-cell receptor β chain genes of PB T-cells. Fourteen ITP patients with H pylori infection and 12 ageadjusted healthy volunteers were studied. Of the 14 patients, 8 patients (responders) exhibited a platelet response after successful H pylori eradication therapy, but 6 patients (nonresponders) did not. V±5.2, V±15, and V±19 gene usage by clonally expanded T-cells in PB obtained before H pylori eradication therapy was significantly higher in responders than in nonresponders or healthy volunteers (V±5.2, P = .023; V±15, P = .004; V±19, P = .036). Furthermore, an abrogation of clonally expanded T-cells was observed after therapy in some responders.These findings suggest that specific T-cell clones accumulate in H pylori-related ITP and that such clones may be associated with immune-mediated destruction of platelets.