The vitreous levels of the angiogenic polypeptide vascular endothelial growth factor (also known as vascular permeability factor) were measured and compared in eyes with and without proliferative diabetic retinopathy. Undiluted vitreous samples from 20 eyes were collected at the time of vitrectomy, and vascular endothelial growth factor levels were determined by using a time-resolved immunofluorometric assay. Vitreous vascular endothelial growth factor levels were significantly higher in eyes with proliferative diabetic retinopathy than in eyes without proliferative diabetic retinopathy (P = .006; Wilcoxon Rank Sum Test). The median vitreous concentration in the eyes with proliferative diabetic retinopathy was 29.1 pM and exceeded the known concentration required for the maximal proliferation of vascular endothelial cells in vitro. These data are consistent with vascular endothelial growth factor serving as a physiologically relevant angiogenic factor in proliferative diabetic retinopathy.