Background— Prolyl hydroxylase domain (PHD) proteins, including PHD1, PHD2, and PHD3, mediate oxygen-dependent degradation of hypoxia-inducible factor (HIF)-α subunits. Although angiogenic roles of hypoxia-inducible factors are well known, the roles of PHDs in the vascular system remain to be established. Methods and Results— We evaluated angiogenic phenotypes in mice carrying targeted disruptions in genes encoding different PHD isoforms. Although Phd1 −/− and Phd3 −/− mice did not display apparent angiogenic defects, broad-spectrum conditional knockout of Phd2 led to hyperactive angiogenesis and angiectasia. Blood vessels in PHD2-deficient mice were highly perfusable. Furthermore, examination of medium-sized vessels in subendocardial layer in the heart demonstrated successful recruitment of vascular smooth muscle cells. Surprisingly, increased vascular growth was independent of local efficiency of Phd2 disruption. Mice carrying significant Phd2 disruption in multiple organs, including the liver, heart, kidney, and lung, displayed excessive vascular growth not only in these organs but also in the brain, where Phd2 disruption was very inefficient. More surprisingly, increased accumulation of hypoxia-inducible factor-1α and angiectasia in the liver were not accompanied by corresponding increases in hepatic expression of Vegfa or angiopoietin-1 . However, the serum vascular endothelial growth factor-A level was significantly increased in PHD2-deficient mice. Conclusions— PHD2, but not PHD1 and PHD3, is a major negative regulator for vascular growth in adult mice. Increased angiogenesis in PHD2-deficient mice may be mediated by a novel systemic mechanism.