We report a 60-year-old woman with idiopathic thrombocytopenic purpura who experienced acute infarction of the middle cerebral artery. She was treated with an antiplatelet agent and prednisolone to limit platelet activation and destruction. In parallel with clinical amelioration, levels of plasma platelet microparticles (PMPs), a procoagulant factor in platelet activation, decreased after treatment but increased after reduction of the prednisolone dose, resulting in progression of vascular stenosis. Immunosuppressive therapy with cyclosporine normalized plasma PMP levels, and no additional vascular events occurred during the 3-month follow-up period. Immunosuppressive therapy to decrease plasma PMP levels is warranted after acute ischemic stroke in the context of idiopathic thrombocytopenic purpura.