Twenty-two patients on regular hemodialysis treatment suffering from renal anemia were treated with intravenous recombinant human erythropoietin (rhEPO) for more than 8 weeks. Before and 4 and 8 weeks after the start of rhEPO administration, we measured prothrombin time, activated partial thromboplastin time, fibrinogen (FBG), antithrombin III activity (ATIII), plasminogen activity (PLG), alpha 2-plasmin inhibitor activity (alpha 2 PI), alpha 2-plasmin inhibitor-plasmin complex (alpha 2 PIC), and cross-linked fibrin degradation products (XL-FDP) in citrated plasma to determine whether rhEPO treatment enhances coagulation and fibrinolytic activity. The pretreatment values of FBG, alpha 2 PIC, and XL-FDP were significantly higher than the normal control values. The pretreatment values of ATIII, PLG, and alpha 2 PI were significantly lower than the normal control values. Platelet count and FBG were significantly increased 4 and 8 weeks after treatment with rhE-PO. The prothrombin time was significantly shortened 8 weeks after rhEPO treatment, but the activated partial thromboplastin time did not change. PLG was significantly decreased 4 and 8 weeks after rhEPO treatment, and ATIII and alpha 2 PI were significantly decreased 8 weeks after rhEPO treatment. alpha 2 PIC was significantly increased 8 weeks after rhEPO treatment, and XL-FDP was significantly increased 4 and 8 weeks after rhEPO treatment. These data suggest that in patients on regular hemodialysis treatment coagulation and fibrinolysis are already enhanced before the start of rhEPO treatment and that rhEPO administration further enhances these disorders.