To investigate the pathophysiological role of thrombopoietin (TPO) in thrombopoiesis, we measured its serum levels in 15 healthy individuals, 84 patients with various hematological diseases and 2 patients with liver cirrhosis using an enzyme immunoassay procedure. The TPO level was 0.84 ± 0.40 f mol/ml in normal individuals. TPO levels were considerably elevated in patients with myelosuppression after intensification chemotherapy of acute leukemia in complete remission (postchemotherapy group; n = 18; 18.46 ± 9.70 f mol/ml). When the data of normal individuals and the postchemotherapy group were combined, TPO levels were inversely correlated with the platelet count in this combined group. We compared these data of normal individuals and the postchemotherapy group with various hematological disease states. In aplastic anemia (n = 13; 16.03 ± 9.44 f mol/ml), acute lymphoblastic leukemia (n = 5; 10.36 ± 5.57 f mol/ml), malignant lymphoma (n = 6; 2.79 ± 2.27 f mol/ml), multiple myeloma (n = 3; 3.34 ± 0.20 f mol/ml) and chronic lymphocytic leukemia (n = 2; 1.71 ± 3.91 f mol/ml), the relationship of serum TPO levels and platelet counts was almost the same as in the combined group with normal individuals and the postchemotherapy group. However, the TPO levels were slightly higher in myeloproliferative disorders (n = 12; 1.99 ± 1.47 f mol/ml) and lower in acute myelogenous leukemia (n = 8; 2.27 ± 1.25 f mol/ml), hypoplastic leukemia (n = 3; 2.76 ± 2.23 f mol/ml), myelodysplastic syndrome (n = 2; 0.42 ± 0.60 f mol/ml), liver cirrhosis (n = 2; 1.50 ± 0.92 f mol/ml) and idiopathic thrombocytopenic purpura (n = 12; 2.08 ± 1.41 f mol/ml), when compared to the regression line for the combined group with normal individuals and postchemotherapy group. These findings suggest that TPO might play an important role in regulation of the platelet count in normal and pathological conditions.