Plasma levels of the vascular endothelial cell markers, thrombomodulin (TM), plasminogen activator inhibitor-1 (PAI-1), tissue type plasminogen activator (t-PA), and von Willebrand factor (vWF) were measured in 55 patients on maintenance haemodialysis (HD). TM, PAI-1 and vWF antigen levels were significantly increased in patients before HD, but t-PA antigen was not Compared with levels before HD, t-PA and vWF antigens were significantly increased 1 h after HD and at the end of HD. TM antigen was significantly increased 1 h after HD, and plasma PAI-1 antigen was decreased at the end of HD. TM and vWF antigen levels were negatively correlated with the time (years) on HD. It is concluded that HD may cause endothelial cell damage and that the increases in plasma TM, PAI-1 and vWF levels before HD, and the decrease in the release of TM and vWF antigens from vascular endothelial cells, might be caused by vascular endothelial cell damage from long-term HD.