Localization of the site of blood-retinal barrier breakdown in diabetes has been controversial. It has been particularly difficult to make assessments in clinical material where the use of tracer materials may not be practical. In this study, immunohistochemical staining for albumin was performed on paraffin-embedded eyes from patients with no known ocular disease and those with various stages of diabetic retinopathy. No extravascular albumin was detected in the retina or retinal pigment epithelium (RPE) of normal nondiabetics or diabetics with no ocular findings, but it was detected in 12.5% of mildly affected diabetics, 20% of background diabetic retinopathy cases, and 89% of proliferative diabetic retinopathy cases. The inner retinal vasculature appeared to be the primary site of leakage in diabetics because all cases demonstrating extravascular albumin-positivity expressed it in the inner retina. It usually permeated the vessel walls and spread along the inner surface of the retina. Some of these cases also contained albumin in the outer retina and RPE, suggesting that additional leakage also may occur through the RPE. A case of cytomegalovirus (CMV) retinitis showed albumin staining predominantly in the inner retina, whereas a rhegmatogenous retinal detachment showed only outer retina staining. These data suggest immunohistochemical staining for albumin may be a useful technique for localizing blood-retinal barrier breakdown.