Abstract： Objective To evaluate the role of thrombopoietin (TPO) in the pathology of chronic thrombocytopenic disease.Methods We measured the endogenous plasma concentration of TPO in 40 patients with acquired aplastic anaemia (AA) and in 32 patients with idiopathic thrombocytopenic purpura (ITP) by a sensitive Sandwich enzyme-linked immunosorbent assay (ELISA) and compared the results.Results Plasma TPO concentrations were significantly higher in AA patients (774±393 pg/ml) in comparison with healthy control subjects (55±34 pg/ml, P<0.001), and there was a significant negative correlation between their plasma TPO concentrations and platelet counts. In patients with ITP, however, the TPO levels were normal or only slightly elevated (73±36 pg/ml), and there was no significant difference compared with healthy controls (P>0.05). There was also no relationship between their plasma TPO levels and platelet counts.Conclusions TPO levels may be regulated not only by platelets but also by megakaryocytes in AA and ITP, and measurement of TPO levels is useful for diagnosing thrombocytopenia and understanding the pathophysiology of thrombocytopenia.