Immune thrombocytopenic purpura (ITP) in childhood is a heterogeneous clinical disorder characterized by immune-mediated platelet destruction. Although generally considered to involve autoreactive B lymphocytes which produce antiplatelet antibodies, there is increasing evidence that T lymphocytes also play an important role in this autoimmune process. We studied 11 children with acute ITP and 19 children with chronic ITP and observed elevated numbers of TCRγδ+ T lymphocytes in several patients. In the three children with the highest elevations (TCRγδ+/CD3+ percentage ranging from 37.8 to 48.1% at initial evaluation), the expanded cell population exclusively expressed the surface Vδ2/Vγδ heterodimer and had enhanced,in vitro proliferation to mycobacterial extracts and IL-2. Analysis of the nucleotide sequences used by these TCRγδ+ cells demonstrated a diverse set of VDDJC gene rearrangements, indicating polyclonal expansion of cells reminiscent of a superantigen response. There was a close correlation between the number of TCRγδ+ T lymphocytes and the degree of thrombocytopenia in each patient. TCR78+ T lymphocytes may be important in the pathogenesis of immunemediated platelet destruction in some children with ITP.