Hemostatic abnormalities were examined in 55 patients during maintenance hemodialysis (HD). Before HD, plasma protein C and protein S antigens were almost within the normal range, while plasma thrombin-antithrombin III complex (TAT III) and plasmin-plasmin inhibitor complex (PPIC) levels in HD patients were increased slightly, and plasminogen activator inhibitor 1 level was significantly increased, compared to that in normal volunteers. Plasma activated protein C (APC) and protein C inhibitor (PCI) complex and APC alpha 1 antitrypsin (alpha 1AT) complex were not detected in normal volunteers; however, plasma APC-PCI complex was increased in 36 of the patients and plasma APC-alpha 1AT complex was increased in 25 patients. Plasma PCI levels in these patients before HD were significantly decreased. Plasma TAT, PPIC, and tissue type plasminogen activator levels were significantly higher before HD than after 1 hour HD and at the end of HD, while the changes in plasma protein C antigen, protein S antigen, PCI antigen, APC-PCI complex, and APC-alpha 1AT complex were not significant after 1 hour of HD or at the end of HD compared to levels before HD. Plasma PCI levels were correlated with APC-PCI complex, suggesting that decreased PCI levels might be caused by the activation of protein C.