The effect of plateletpheresis on endothelium, which has strong effects on blood coagulation, fibrinolysis and platelet function, is not known. Activation of leukocytes and subsequent generation of proinflammatory cytokines during the extracorporeal circulation may activate the endothelium. To test this hypothesis we measured plasma levels of tumor necrosis factor (TNF)-alpha as a prototype of the proinflammatory cytokines, and von Willebrand factor (vWF) and fibronectin as endothelial release/damage markers before and after a single plateletpheresis procedure on an intermittent-flow machine Haemonetics MCS 3p in 17 healthy donors. We found a significant increase in median plasma level of TNF-alpha following plateletpheresis (3.5 vs 26.5 pg/ml, P=0.02). Such increases in vWF and fibronectin were not observed. The increase in plasma TNF-alpha indicates that a single plateletpheresis procedure causes leukocyte activation which does not seemingly impair endothelial cell function. The relation of plateletpheresis-induced proinflammatory cytokine release to some adverse effects observed in both donors and recipients, and the effect of repeated plateletpheresis on endothelium deserve further studies.