Recent studies have identified factors responsible for angiogenesis within developing tumors, but mediators of vessel formation at sites of trauma, injury, and wound healing are not clearly established. Here we show that sphingosine 1‐phosphate (S1P) released by platelets during blood clotting is a potent, specific, and selective endothelial cell chemoattractant that accounts for most of the strong endothelial cell chemotactic activity of blood serum, an activity that is markedly diminished in plasma. Preincubation of endothelial cells with pertussis toxin inhibited this effect of S1P, demonstrating the involvement of a G_αi‐coupled receptor. After S1P‐induced migration, endothelial cells proliferated avidly and differentiated forming multicellular structures suggestive of early blood vessel formation. S1P was strikingly effective in enhancing the ability of fibroblast growth factor to induce angiogenesis in the avascular mouse cornea. Our results show that blood coagulation initiates endothelial cell angiogenic responses through the release of S1P, a potent endothelial cell chemoattractant that exerts its effects by activating a receptor‐dependent process.—English, D., Welch, Z., Kovala, A. T., Harvey, K., Volpert, O. V., Brindley, D. N., Garcia, J. G. N. Sphingosine 1‐phosphate released from platelets during clotting accounts for the potent endothelial cell chemotactic activity of blood serum and provides a novel link between hemostasis and angiogenesis. FASEB J. 14, 2255–2265 (2000)