A venous loop developed in a patient with proliferative diabetic retinopathy. Fluorescein angiography showed staining of the vessel wall and nonperfusion of the surrounding capillary bed. Light microscopy and scanning and transmission electron microscopy showed the loop to consist of a telangiectatic retinal vein that had passed through a discontinuity in the internal limiting membrane. The thin-walled vessel loop was lined by an attenuated endothelium and occasional pericytes. The vitreous was detached posteriorly except at the venous loop and a few other points at which there was tenting of the internal limiting membrane. These findings suggested that vitreous traction plays a role in the pathogenesis of these venous loops.