Proliferative diabetic retinopathy is thought to be mediated by the hypoxic regulation of angiogenic growth factors, in particular the vascular endothelial growth factor (VEGF) family. The aim of this study was to determine if placental growth factor (PIGF), a recently identified member of the VEGF family, was expressed in diabetic eyes undergoing preretinal neovascularization. Rabbit anti-PIGF antiserum was raised using a 20-amino acid N-terminal sequence to PIGF and did not cross react with VEGF165. Immunohistochemistry was performed on specimens of normal retina (n = 8), diabetic retina in the absence (n = 7) and presence (n = 4) of proliferative retinopathy, scatter laser-treated diabetic retina (n = 7), excised fibrovascular preretinal membranes (n = 12), and nondiabetic fibrocellular epiretinal (n = 7) membranes. PIGF levels were also determined in vitrectomy specimens from patients with either proliferative diabetic retinopathy or macular hole. PIGF immunoreactivity was intensely localized to the endothelial and perivascular regions of newly formed blood vessels of excised fibrovascular preretinal membranes. Intense localization of PIGF protein was also observed in superficial retinal vessels in diabetic retinae adjacent to neovascular preretinal membranes. Localization of PIGF was weak or absent in diabetic retinae that showed no evidence of neovascular proliferation. PIGF protein was also absent in normal retinae, in diabetic retinae that had received extensive treatment with scatter laser photocoagulation, and in nonvascularized epiretinal membranes. PIGF was present in all diabetic vitreous samples (mean 103 pg/ml) but nondetectable in control samples. These results strongly implicate a role for PIGF in the pathogenesis of proliferative diabetic retinopathy.