We determined plasma levels of thrombomodulin, thrombin-antithrombin III complex (TAT), protein C, protein S, and plasmin-α2 plasmin inhibitor complex (PIC) before and after hemodialysis in 54 patients receiving chronic hemodialysis, to evaluate the blood-coagulation system and to evaluate the antithrombogenicity of various dialyzer membranes. Predialysis levels of thrombomodulin and TAT were both significantly increased compared with normal control values, but levels of protein C, protein S, and PIC were not changed. In patients dialyzed with ethylene vinyl alcohol (EVAL) and polysulfone membranes, postdialysis levels of thrombomodulin, TAT, protein C, protein S, and PIC were not significantly different from the predialysis levels. However, in patients dialyzed with regenerated cellulose and polymethylmethacrylate (PMMA) membranes, postdialysis levels of thrombomodulin, TAT, and PIC were significantly higher than predialysis levels. We conclude that patients on maintenance hemodialysis were considered to be in a state of hypercoagulability before hemodialysis, and a single hemodialysis session using regenerated cellulose and PMMA membrane may have caused injury to vascular endothelial cells, hypercoagulability, and enhancement of fíbrinolytic activity.