Platelet microparticles (PMPs) are vesicles derived from platelet membranes that are too small (less than 0.5 micron) to be detected in routine platelet counting. They arise in association with platelet activation and other unknown causes. Elevated PMPs have been observed in idiopathic thrombocytopenic purpura (ITP), a disorder in which autoantibody interacts with platelets and the opsonized platelets are destroyed by macrophages. However, the clinical significance of PMP has been unknown. Using flow cytometry, we examined PMP concentrations in 62 patients with ITP and in 33 normal control subjects to assess the clinical significance of PMP in ITP. When compared with PMP levels in control subjects, PMP levels were significantly higher (p less than 0.005) in patients with ITP, but considerable variation among individual patients was observed. Patients with platelet counts less than or equal to 60,000 were evaluated for correlation of PMP levels with manifestations of thrombocytopenias; patients without symptoms (free of petechiae or mucosal bleeding) are found to have significantly higher PMP levels (p less than 0.05) than patients with symptoms, suggesting hemostatic protection by PMP. Additionally, we identified a group of patients with ITP who experienced neurologic complications resembling transient cerebral ischemic attacks (TIAs): recurrent episodes of dizzy spells or weakness in mild cases, and coma, seizure, or progressive dementia in advanced cases. Small cerebral infarcts were demonstrated by computed axial tomography scan or magnetic resonance imaging in spite of severe thrombocytopenias. Patients with this syndrome are often found to have higher PMP levels (p less than 0.005) when compared with the group free of neurologic complications. It is concluded that PMPs play an important role in hemostasis in patients with thrombocytopenia, and that high concentrations of hemostatically active PMP can be thrombogenic in certain clinical settings. Quantitation and characterization of PMP is important in assessment and management of patients with thrombocytopenia.