Sphingosine 1-phosphate (S1P), a bioactive lipid, is produced and stored in platelets and is released from activated platelets during blood coagulation activation. Thrombin, which is also generated during blood coagulation, has been shown to induce tissue factor (TF), the initiator of blood coagulation, in endothelial cells (ECs); however, the effect of S1P on this process is not evaluated. Here we demonstrated that S1P strongly potentiated thrombin-induced TF expression in ECs and that S1P itself did not induce TF expression. Among signaling lipids, platelet-activating factor slightly enhanced thrombin-induced TF expression; other lipids, including lysophosphatidic acid, lysophosphatidylcholine, sphingosine, and C₂-ceramide exert no effect on TF expression. S1P enhanced TF expression at the transcriptional level, possibly via promoting the activation of transcription factors nuclear factor–κB (NF-κB) and Egr-1. Thrombin weakly and S1P strongly activated extracellular signal–regulated kinase 1/2 (ERK1/2) mitogen-activated protein (MAP) kinase and, in the presence of both stimulants, enhanced and sustained activation of this kinase was observed. The ERK1/2-specific inhibitor PD98059 significantly inhibited enhanced TF expression induced by both stimulants but only weakly inhibited thrombin-induced TF expression, thus indicating the requirement of the ERK1/2 pathway in synergistic induction of TF expression. In addition, we found that thrombin and S1P rapidly up-regulated the expression of S1P receptors, endothelial differentiation gene-1 (EDG-1) and EDG-3, thereby suggesting that the effect of S1P on TF expression and other EC functions may be enhanced by thrombin and S1P itself. The present data reveal the synergistic effect of S1P on thrombin-induced TF expression in ECs, which may promote further thrombin and S1P generation, thus propagating a positive feedback reaction.