Reliable and valid quantification of functional status of the corneal endothelium is a desirable input to optimize monitoring and intervention strategies in clinical disorders affecting endothelium. We examined the validity and clinical usefulness of an in vivo corneal stress test in an age and sex matched sample of 15 eyes each of normal controls, pseudophakic corneal edema and Fuchs' dystrophy. The test measures Percentage Recovery Per Hour (PRPH) as endothelial function by evaluating the deswelling response of cornea. In pseudophakic corneal edema assessed PRPH is significantly lower than in normal cornea (p < 0.01). At optimum critical cut off PRPH of 47.5%, the test has high specificity and sensitivity (93.3%) in pseudophakic corneal edema eyes, which validates the test. When baseline corneal thickness is more than 0.55 mm, PRPH is determined to be below the cut-off value in 100% of eyes. In eyes where baseline corneal thickness is less than 0.55 mm, PRPH ranges from 26.7% to 76.8%. We conclude that the in vivo corneal stress test quantifies endothelial function reliably and when compared to age specific normal eyes, it helps in identifying and monitoring low endothelial function.