The vitreous levels of angiogenin, which is a potent blood vessel-inducing protein, were measured to determine their association with proliferative diabetic retinopathy (PDR). Undiluted vitreous fluid specimens were collected from 30 eyes with PDR at the time of vitrectomy. A sandwich enzyme-liked immunosorbent assay was then used to quantitate the levels of angiogenin. As a control we determined the levels in the specimens from 21 patients with proliferative vitreoretinopathy (PVR) and 4 patients with idiopathic macular epiretinal membrane (IERM). The average angiogenin level in the eyes with PDR was 43.7 ng/ml, and no significant difference was observed among PDR, PVR and their reoperation cases. In the category of IERM, the mean concentration of angiogenin was 2.1 ng/ml, which was significantly lower than that of the PDR and PVR cases. Our study thus demonstrated a significant increase in the vitreous angiogenin levels in eyes with PDR, PVR and those undergoing reoperation for these conditions in comparison to eyes with IERM. We therefore postulated that the elevated angiogenin levels thus reflected a breakdown of the blood-ocular barrier in eyes with PDR and PVR.