Summary Objective: circulating plasma thrombomodulin (TM), tissue-type plasminogen activator (t-PA) and urokinase-type plasminogen activator (u-PA) are endothelial cell markers which may reflect endothelial injury. To find out to what extent diagnostic cardiac percutaneous catheterisation irritates vascular endothelium we conducted a prospective study in 114 children. Results: compared to starting values TM, t-PA and u-PA concentrations show a clearly significant increase at the end of cardiac catheterisation. TM and u-PA returned to pretreatment values 24 hours later, t-PA remained elevated. Four of 114 children developed vascular occlusion near the puncture sites within 48 hours of cardiac catheterisation. Two patients < 1 year of age showed resistance to APC, one child with recurrent thromboembolism showed familial thrombophilia with elevated lipoprotein (a) levels. Conclusions: data of this study indicate that increased TM, t-PA and u-PA concentrations after percutaneous cardiac catheterisation in children are signs of endothelial damage.