Tumor necrosis factor alpha (TNFα) induces prothrombotic effects in endothelial cells in vitro. Treatment of endothelial cells with TNFα (5 ng/mL, 4 hours) led to increased superoxide release as assessed by cytochrome c reduction (a) and upregulation of P-selectin on the surface membrane (b) in human umbilical vein endothelial cells. Additionally, TNFα decreased thrombomodulin (c) and increased plasminogen activator inhibitor 1 (PAI-1) (d) as measured by real-time polymerase chain reaction in human microvascular endothelial cells (HMECs). (e) TNFα elevated tissue factor expression as well as protein on the cell surface membrane. (f) These changes resulted in significant acceleration of blood clotting time induced by lysates of TNFα-treated HMECs. (g-j) The supernatant of TNFα (5 ng/mL, 4 hours)-treated HMECs was used to stimulate untreated endothelial cells. This resulted in an upregulation of P-selectin after 30 minutes and prothrombotic changes in mRNA levels of thrombomodulin, PAI-1, and tissue factor which were qualitatively similar to, but less than those achieved by direct TNFα stimulation, indicating that autocrine factors secreted by TNFα-stimulated HMECs contribute to the prothrombotic phenotype. *Significantly different versus control at P <0.05 (n = 6 to 8). MFI, mean fluorescence intensity; rel. to ctr., relative to control.