Corneal epithelial permeability was determined using an automatic fluorophotometer in 80 consecutive diabetic patients. The corneal epithelium, which is the most important part of the corneal diffusion barrier, showed a significant increase of permeability to watery, ionic substances (natrium fluorescein). The permeability was increased 5.4 times in comparison with nondiabetic individuals and a distinct correlation of increased permeability with the presence and duration of retinal nonperfusion areas was observed.