Regular consumption of low-dose aspirin reduces the occurrence of colorectal, esophageal, stomach, and gastrointestinal cancers. The underlying mechanism is unknown but may be linked to inhibition of angiogenesis. Because the effective doses of aspirin are consistent with the inhibition of cyclooxygenase-1 in platelets, we used liquid chromatography with tandem mass spectrometry analyses and immunoassays of human platelet releasates coupled with angiogenesis assays to search for the mediators of these effects. Blood or platelet-rich plasma from healthy volunteers stimulated with platelet activators produced a broad range of eicosanoids. Notably, preincubation of platelets with aspirin, but not with a P2Y 12 receptor antagonist, caused a marked reduction in the production of 11-hydroxyeicosatetraenoic acid (HETE) and 15 (S)-HETE, in addition to prostanoids such as thromboxane A 2. Releasates from activated platelets caused cell migration and tube formation in cultured human endothelial cells and stimulated the sprouting of rat aortic rings in culture. These proangiogenic effects were absent when platelets were treated with aspirin but returned by coincubation with exogenous 15 (S)-HETE. These results reveal 15 (S)-HETE as a major platelet cyclooxygenase-1 product with strong proangiogenic effects. Thus, 15 (S)-HETE represents a potential target for the development of novel antiangiogenic therapeutics, and blockade of its production may provide a mechanism for the anticancer effects of aspirin.—Rauzi, F., Kirkby, NS, Edin, ML, Whiteford, J. Zeldin, DC, Mitchell, JA, Warner, TD Aspirin inhibits the production of proangiogenic 15 (S)-HETE by platelet cyclooxygenase-1.