Diabetic macular edema, resulting from increased microvascular permeability, is the most prevalent cause of vision loss in diabetes. The mechanisms underlying this complication remain poorly understood. In the current study, diabetic vascular permeability (blood-retinal barrier breakdown) is demonstrated to result from a leukocyte-mediated Fas-FasL-dependent apoptosis of the retinal vasculature. Following the onset of streptozotocin-induced diabetes, FasL expression was increased in rat neutrophils (P<0.005) and was accompanied by a simultaneous increase in Fas expression in the retinal vasculature. Static adhesion assays demonstrated that neutrophils from diabetic, but not control, rats induced endothelial cell apoptosis in vitro (P<0.005). The latter was inhibited via an antibody-based FasL blockade (P<0.005). In vivo, the inhibition of FasL potently reduced retinal vascular endothelial cell injury, apoptosis, and blood-retinal barrier breakdown (P<0.0001) but did not diminish leukocyte adhesion to the diabetic retinal vasculature. Taken together, these data are the first to identify leukocyte-mediated Fas-FasL-dependent retinal endothelial cell apoptosis as a major cause of blood-retinal barrier breakdown in early diabetes. These data imply that the targeting of the Fas-FasL pathway may prove beneficial in the treatment of diabetic retinopathy.