Background The rate of measured glomerular filtration rate (GFR) change in kidney donor years after donation has not been adequately addressed. Whether this change is accelerated in the setting of 1 kidney is also understudied. Methods Two hundred fourteen randomly selected donors underwent serial GFR measurements of nonradioactive iohexol. Estimated GFR at each visit was calculated using the Chronic Kidney Disease Epidemiology Collaboration and Modification of Diet in Renal Disease study equations. Results Glomerular filtration rate visits were 4.8 ± 1.3 years apart and the second occurring 16.9 ± 9.1 years after donation. Most (97.7%) were white, 60.8% female, and 78.5% were related to their recipient. Most, 84.6%, had a GFR of 60 mL/min per 1.73 m 2 or higher, 14.0% had a GFR between 45 and 60 mL/min per 1.73 m 2 , and 1.4% had a GFR less than 45 mL/min per 1.73 m 2 . Between visits 1 and 2, 56.5% had a GFR decline, 36.0% increase, and in 7.5%, there was no change. Overall, GFR declined at a rate of -0.42 mL/min per 1.73 m 2 per year. Of GFR estimating models, only Chronic Kidney Disease Epidemiology Collaboration-Creatinine equation produced a slope that was steeper than measured GFR. Conclusions Nearly 2 decades postdonation GFR declined at a rate similar to that seen in the general population, and in one third, GFR continues to increase.