The aim of this study was to evaluate the relation between thrombopoietin (TPO) and thrombocytopenia in patients with liver cirrhosis and those with idiopathic portal hypertension (IPH) before and after partial splenic embolization (PSE). We examined changes in platelet counts, liver function, megakaryocyte function, and plasma TPO levels after PSE in 30 patients (20 with liver cirrhosis, and 10 with IPH). Platelet counts in both cirrhosis and IPH increased significantly 2 months after PSE (cirrhosis group, 4.0+/-1.9 vs. 7.5+/-4.4x10(4)/&mgr;l: P=0.0002; IPH group, 4.0+/-1.7 vs. 6.5+/-2.3x10(4)/&mgr;l: P=0.0042). Plasma TPO level and prothrombin time increased significantly and alanine aminotransferase level (ALT) and total bilirubin level decreased significantly 2 months after PSE in the cirrhosis group (plasma TPO level, 0.57+/-0.30 vs. 0.72+/-0.27 fmol/ml: P=0.024), but not in the IPH group (0.56+/-0.21 vs. 0.55+/-0.34 fmol/ml: P=0.94). Moreover, the score of megakaryocytes with platelet production, an index of platelet production by megakaryocytes in bone marrow, increased significantly in the cirrhosis group. TPO production in cirrhotic patients is restored after PSE, leading to the resolution of thrombocytopenia. But patients with IPH had no change in liver function, indicating that only decreased spleen volume was responsible for the improvement in platelet count.