Angiogenesis is an inevitable event in tumor progression and metastasis, and thus has been a compelling target for cancer therapy in recent years. Effective inhibition of tumor progression and metastasis could become a promising way to treat tumor-induced angiogenesis. We discovered that a fungus, Neosartorya sp., isolated from a soil sample, produced a new angiogenesis inhibitor, which we designated azaspirene. Azaspirene was previously shown to inhibit human umbilical vein endothelial cell (HUVEC) migration induced by vascular endothelial growth factor (VEGF) at an effective dose, 100% of 27 micromol/L without significant cell toxicity. In the present study, we investigated the antiangiogenic activity of azaspirene in vivo. Azaspirene treatment reduced the number of tumor-induced blood vessels. Administration of azaspirene at 30 microg/egg resulted in inhibition of angiogenesis (23.6-45.3% maximum inhibition relative to the controls) in a chicken chorioallantoic membrane assay. Next, we elucidated the molecular mechanism of antiangiogenesis of azaspirene. We investigated the effects of azaspirene on VEGF-induced activation of the mitogen-activated protein kinase signaling pathway in HUVEC. In vitro experiments indicated that azaspirene suppressed Raf-1 activation induced by VEGF without affecting the activation of kinase insert domain-containing receptor/fetal liver kinase 1 (VEGF receptor 2). Additionally, azaspirene preferentially inhibited the growth of HUVEC but not that of the non-vascular endothelial cells NIH3T3, HeLa, MSS31, and MCF-7. Taken together, these results demonstrate that azaspirene is a novel inhibitor of angiogenesis and Raf-1 activation that contains a unique carbon skeleton in its molecular structure.