CD40-ligand (CD154) is expressed on activated CD4+ T lymphocytes and is essential for the T cell-dependent activation of B lymphocytes. CD154 is also expressed at the activated platelet surface. In this study, we show that platelet-associated CD154 is increased in immune thrombocytopenic purpura (ITP), a disease characterized by an autoimmune response against proteins of the platelet membrane. CD154 and its messenger RNA were also present in increased amounts in the megakaryocytes of patients with ITP. We found that platelet-associated CD154 is competent to induce the CD40-dependent proliferation of B lymphocytes, and we observed an in vitro CD154-dependent production of antibodies to the GPIIb/IIIa complex (integrin alphaIIbbeta3) when platelets and peripheral blood B lymphocytes from ITP patients with circulating anti-GPIIb/IIIa antibody were cultured together. Therefore, platelet-associated CD154 expression is increased in ITP and is able to drive the activation of autoreactive B lymphocytes in this disease.