Objective: In vitro incubation of cultured endothelial cells under hypoxia leads to the activation of these cells and results in an increase of their adhesiveness for neutrophils (PMN). Because of the possible relevance of these observations for pathological situations, we investigated whether adherence of PMN also occurs in an entire vein after its incubation in hypoxic conditions. Methods: Human umbilical veins in complete cords were incubated for 2 h in normoxic or hypoxic conditions and the adherence of unstimulated human 51Cr-labelled-PMN was measured under flow conditions. Experiments with human umbilical vein endothelial cells (HUVEC) were performed in parallel for comparison. Morphological studies in scanning electron microscopy were carried out in both in vitro and ex vivo situations. Results: Hypoxia induced an increase in the adherence of PMN either to HUVEC or to the umbilical vein endothelium up to 5- to 6-fold when compared to normoxic conditions (P < 0.001). In both cases, this hypoxia-induced adherence was inhibited by anti-ICAM-1 antibodies or when the PAF (platelet-activating factor) synthesis was blocked during hypoxia by oleic acid. Furthermore, the adherence of PMN was inhibited when PMN were pre-incubated with WEB 2086 (a selective PAF receptor antagonist). These results indicate a crucial role of PAF in this process. Morphological studies confirmed that the number of PMN adherent to hypoxic HUVEC or to the hypoxic umbilical vein endothelium was much greater than the number of PMN on normoxic endothelial cells. Both in vitro and ex vivo, PMN adherent to the hypoxic endothelial cells to the contrary of the ones adherent to normoxic endothelial cells demonstrated membrane foldings typical of an activated state. Conclusion: These results show that in a complete vein, hypoxia induced an increased adhesiveness of endothelial cells for PMN by a similar mechanism to the one observed for cultured endothelial cells. They suggest an active role of endothelial cells in the initiation of the inflammatory response often described in ischemic-reperfused organs.