Tumor necrosis factor-α (TNF-α) mediates proinflammatory responses in primary human umbilical vein endothelial cells (HUVECs), and it upregulates the expression of secretory group IIA phospholipase A(2) (sPLA(2)-IIA). sPLA(2)-IIA plays a pivotal role in inflammation, and antithrombin (AT) possesses properties that are beneficial to endothelial cells. Therefore, we investigated the effects of AT on the expression of sPLA(2)-IIA in TNF-α-stimulated HUVECs. TNF-α potently upregulated the expression of sPLA(2)-IIA, and prior treatment of cells with AT inhibited the expression of sPLA(2)-IIA in HUVECs. Also, antibodies or siRNA for syndecan-4 blocked the protective effect of AT. Furthermore, PI3-kinase and the AKT pathway are significantly involved in the AT-mediated inhibition of the expression of sPLA(2)-IIA. These results show that AT effectively suppresses the upregulated sPLA(2)-IIA expression, which might contribute to the cytoprotective effects of AT in the treatment of severe inflammatory diseases.