Treatment with intravenous recombinant human interleukin-2 (rh IL-2) is frequently accompanied by the capillary leak syndrome and disturbances of the coagulation system. Although the exact mechanisms are still not fully understood, the involvement of the endothelium is proven. This investigation aimed to elucidate more precisely the role of the endothelium in the generation of IL-2-based side-effects. In nine tumour patients receiving intravenous rh IL-2, parameters characterizing endothelial cell activation as well as activation of the coagulation system were evaluated. A significant increase of the circulating endothelial leucocyte adhesion molecule-1 (cELAM-1) and the vasoconstrictor peptide endothelin-1 (ET-1) was observed (P<0.05), indicating activation of endothelial cells. The simultaneous increase of tissue-plasminogen activator and plasminogen activator inhibitor type-1 during therapy (P<0.05) corroborated this observation. A decrease in platelet count parallelled by an increase of fibrin degradation products, the prolongation of partial thromboplastin time, and the decrease of fibrinogen (P<0.05) suggested the development of disseminated intravascular coagulation (DIC), induced by activated endothelium and intensified by transient hepatic failure. We concluded that activation of the endothelium mediated by IL-2 was accompanied by a loss of endothelial integrity and capillary leak. The activated endothelium can trigger DIC via activation of the coagulation cascade. The increased ET-1 might act as an endogenous counter-regulator of the disadvantageous haemodynamic side-effects induced by IL-2.