Cyclooxygenase-2 (COX-2) inhibitors reduce angiogenic responses to a variety of stimuli, suggesting that products of COX-2 may mediate critical steps. Here, we show that thromboxane A₂ (TXA₂) is one of several eicosanoid products generated by activated human microvascular endothelial cells. Selective COX-2 antagonists inhibit TXA₂ production, endothelial migration, and fibroblast growth factor-induced corneal angiogenesis. Endothelial migration and corneal angiogenesis are similarly inhibited by a TXA₂ receptor antagonist, SQ29548. A TXA₂ agonist, U46619, reconstitutes both migration and angiogenesis responses under COX-2-inhibited conditions. These findings identify TXA₂ as a COX-2 product that functions as a critical intermediary of angiogenesis.