Background: Hypoxia-inducible factor 1 alpha (HIF-1α), a component of HIF-1, is expressed in human tumors and renders cells able to survive and grow under hypoxic (low-oxygen) conditions. YC-1, 3-(5′-hydroxymethyl-2′-furyl)-1-benzylindazole, an agent developed for circulatory disorders that inhibits platelet aggregation and vascular contraction, inhibits HIF-1 activity in vitro. We tested whether YC-1 inhibits HIF-1 and tumor growth in vivo. Methods: Hep3B hepatoma, NCI-H87 stomach carcinoma, Caki-1 renal carcinoma, SiHa cervical carcinoma, and SK-N-MC neuroblastoma cells were grown as xenografts in immunodeficient mice (69 mice total). After the tumors were 100–150 mm³, mice received daily intraperitoneal injections of vehicle or YC-1 (30 μg/g) for 2 weeks. HIF-1α protein levels and vascularity in tumors were assessed by immunohistochemistry, and the expression of HIF-1-inducible genes (vascular endothelial growth factor, aldolase, and enolase) was assessed by reverse transcription–polymerase chain reaction. All statistical tests were two-sided. Results: Compared with tumors from vehicle-treated mice, tumors from YC-1-treated mice were statistically significantly smaller (P<.01 for all comparisons), expressed lower levels of HIF-1α (P<.01 for all comparisons), were less vascularized (P<.01 for all comparisons), and expressed lower levels of HIF-1-inducible genes, regardless of tumor type. Conclusions: The inhibition of HIF-1α activity in tumors from YC-1-treated mice is associated with blocked angiogenesis and an inhibition of tumor growth. YC-1 has the potential to become the first antiangiogenic anticancer agent to target HIF-1α.