Heat shock protein 90 (Hsp90) is a molecular chaperone involved in maintaining the correct conformation and stability of its client proteins. This study investigated the effects of Hsp90 inhibitors on client protein expression and key cellular functions required for tumor angiogenesis. The benzoquinone ansamycin Hsp90 inhibitors geldanamycin and/or its derivatives 17-allylamino-17-demethoxygeldanamycin (17-AAG) and 17-(dimethylaminoethylamino)-17-demethoxygeldanamycin inhibited production of vascular endothelial growth factor (VEGF)-A by tumor cells and blocked proliferative responses of human endothelial cells at nanomolar concentrations. 17-AAG also significantly reduced endothelial cell migration, tubular differentiation, invasion through Matrigel, and secretion of urokinase-type plasminogen activator at concentrations at or below those that inhibited proliferation. 17-AAG significantly reduced expression of VEGF receptor (VEGFR)-2 and established Hsp90 client proteins in human endothelial cells in vitro as well as in mouse vena cava, mesenteric vessels, and blood vessels within human tumor xenografts in vivo; this was associated with decreased tumor microvessel density. Finally, we showed for the first time that Hsp90 inhibitors also reduce expression of VEGFR-1 on human vascular endothelial cells, VEGFR-3 on lymphatic endothelial cells in vitro, and all three VEGFRs on mouse vasculature in vivo. Thus, we identify Hsp90 inhibitors as important regulators of many aspects of tumor angiogenesis (and potentially lymphangiogenesis) and suggest that they may provide therapeutic benefit not only via direct effects on tumor cells but also indirectly by inhibiting the production of angiogenic cytokines and responses of activated endothelial cells that contribute to tumor progression and metastasis.