Angiogenesis is a process that involves endothelial cell proliferation, migration, invasion, and tube formation, and the inhibition of these processes has implications for angiogenesis-mediated disorders. The purpose of this study was to examine the antiangiogenic efficacy of YD-3 [1-benzyl-3(ethoxycarbonylphenyl)-indazole], a selective thrombin inhibitor, on thrombin-induced endothelial cell proliferation and neoangiogenesis in a murine Matrigel model. First, the effect of YD-3 on angiogenesis was evaluated in vivo using the mouse Matrigel implant model. Plugs treated with 1 and 10 μM of YD-3 inhibited neovascularization induced by thrombin, protease-activated receptor (PAR) 1, and PAR-4, but not by vascular endothelial growth factor, in a concentration-dependent manner over 7 days. These results indicate that YD-3 has specific antiangiogenic activity on thrombin. YD-3 also inhibited (in a concentration-dependent manner) the ability of thrombin, PAR-1, and PAR-4, but not PAR-2, to induce the proliferation of human umbilical vascular endothelial cells, using a [³H]thymidine incorporation assay. YD-3 predominantly inhibited thrombin-induced vascular endothelial growth factor receptor 2 (Flk-1) expression, but not extracellular signal-regulated kinase 1/2 phosphorylation, using Western blot analysis. YD-3 may have benefit in elucidating pathophysiology induced by thrombin-induced angiogenesis.