For the first time, we present measurements of the scattered light intensity of the human cornea to determine the functional state of the corneal endothelium. In 94 contact lens wearers endothelial alterations were measured by means of a newly developed confocal slit scanning video microscope and correlated with photometrically recorded stromal scattered light intensity. Our control group consisted of 26 persons without corneal pathology. It was shown that in persons wearing contact lenses the endothelial cell density was significantly below that of the control group and endothelial cell polymegatism (coefficient of variability of cell size) was significantly greater. Further, in contact lens wearers the level of scattered light intensity was significantly higher than in the control group, and the difference increased with the wearing time. In addition, the variation of the endothelial cell size correlated with the increased scattered light within the stroma, only a low correlation was found between the endothelial cell density and the scattered light intensity, however. From our results it can be assumed that a corneal endothelium with marked polymegatism has a reduced hydration function. Thus, decreased endothelial function can be determined very early and non-invasively by measuring the corneal scattered light intensity. In addition, the scattered light intensity gives topographical information about corneal swelling.