Diabetic retinopathy is a leading cause of visual disturbance in adults. In proliferative diabetic retinopathy, ischemia-induced pathologic growth of new blood vessels often causes catastrophic loss of vision. Besides VEGF, the existence of another potent ischemia-induced angiogenic factor is postulated. Since ischemia-inducible erythropoietin (Epo) has recently been identified its angiogenic properties, we investigated its potential role during retinal angiogenesis in proliferative diabetic retinopathy (PDR). The vitreous Epo level in patients with PDR was significantly higher than that in nondiabetic patients. Multivariate logistic regression analyses indicated that Epo and VEGF were independently associated with PDR and that Epo was more strongly associated with PDR than VEGF. Blockade of Epo inhibits retinal neovascularization in vivo, and inhibits endothelial cell proliferation response to PDR vitreous in vitro. Our data provide strong evidence that erythropoietin is a potent retinal angiogenic factor independent of VEGF and is capable of stimulating ischemia-induced retinal angiogenesis in proliferative diabetic retinopathy. Inhibition of such molecular mechanisms in the retinal angiogenesis could be a new therapeutical strategy in halting or preventing pathologic angiogenesis in diabetic retinopathy.