E rythropoietin (Epo), a hormone known to stimulate bone marrow erythrocyte production, is widely used to treat anemia in patients at risk for vascular disease. However, the effects of Epo on angiogenesis are not well defined. We studied the role of Epo in a mouse model of retinopathy characterized by oxygen-induced vascular loss followed by hypoxia-induced pathological neovascularization. Without treatment, local retinal Epo levels were suppressed during the vessel loss phase. Administration of exogenous Epo prevented both vessel dropout and subsequent hypoxia-induced neovascularization. Early use of Epo also protected against hypoxia-induced retinal neuron apoptosis. In contrast, retinal Epo mRNA levels were highly elevated during the retinopathy neovascular phase. Exogenous late Epo treatment did not protect the retina, but rather enhanced pathological neovascularization. Epo’s early protective effect occurred through both systemic retinal recruitment of proangiogenic bone marrow–derived progenitor cells and activation of prosurvival NF-κB via Epo receptor activation on retinal vessels and neurons. Thus early retinal Epo suppression contributed to retinal vascular instability, and elevated Epo levels during the proliferation stage contributed to neovascularization and disease. Understanding the role of Epo in angiogenesis is critical to timing its intervention in patients with retinopathy or other diseases in which pathological angiogenesis plays a significant role.