The corneal endothelium in diabetics is particularly susceptible to surgical trauma. This is related to the presence of morphological alterations of endothelial cells and to a supposedly concomitant functional failure of the endothelium in these patients. The aim of our study was to evaluate the corneal endothelial function in diabetics. A fluorophotometric study was performed on 3 groups of diabetic subjects without retinopathy, with nonproliferative retinopathy and with proliferative retinopathy compared with an age-matched control group. We evidenced a significant increase in corneal thickness and in passive corneal permeability in diabetics with severe retinal involvement. These findings are not related with a proportional significant increase in endothelial pump function. We suggest an altered regulation in corneal hydration in diabetic patients depending on the functional failure of endothelial cells that is likely to be connected with corneal biochemical modifications caused by an altered glucidic metabolism.