Objective To investigate the role of platelet indices in the differentiation of thrombocytopenia caused by bone marrow diseases and thrombocytopenia caused by non-marrow diseases.Methods Mean platelet volume(MPV),platelet crit(PCT)and platelet size deviation width(PDW)of 76 othromboeytopenia(TP)patients,66 immune thrombocytopenia purpura(ITP)patients,8 secondary thrombocytocritpenia purpura(STP)patients,8 acquired pure megakaryocytic thrombocytopenia purpura(AATP)patients,140 malignant hematological diseases(MHD)(including acute leukemia,aplastic anemia and myelodysplastic syndromes)patients and 100 healthy people were respectively determined by blood cell analyzer Cell-Dyn 3700.Result Compared with control group,the MPV of the patients with MHD significantly decreased,and then the PDW of them increased.For patients with STP,the MVP and PDW markedly increased.The MVP of patients with AATP was normal,and PDW significantly increased.The MPV,PCT and PDW of ITP and TP showed marked difference(P＜0.01).In contrast with the control group,the MPV,PCT and the PDW of patients with hemopathy and patients with ITP were remarkably different(P＜0.01).Compared with the marrow TP and the non-marrow TP or control group,MPV and PDW showed significant difference(P＜0.01).During the effective treatment of patients with ITP,PLT increased and then MPV and PDW decreased.Conclusion The level changes of MPV and PDW,in particular MPV can primarily judge the cause of thrombocytopenia (MPV＜reference value as caused by bone marrow diseases but MPV＞reference value as caused by bone marrow diseases).In addition,during the effective treatment of patients with ITP,with changes of MPV and PDW present 5 days earlier than that of PLT,may used to observe the therapeutic effect.Myelodysplastic syndromedynamic observation of the platelet parameter helps for the differential diagnosis,judgment of the states of the diseases and observation of the effectiveness of therapy of ITP.