The glomerular filtration rate assesses the function of all nephrons, and the single-nephron GFR assesses individual nephron function. This study showed that in healthy adults, the single-nephron GFR remained constant regardless of age, sex, or height (if ≤190 cm). The glomerular filtration rate (GFR) is considered to be the most important assessment of kidney function. In clinical practice, the total GFR, which is the sum of all the single-nephron GFRs, is determined. Previous studies that have counted glomeruli at autopsy have shown considerable variation in the number of nephrons (210,000 to 2,700,000 nephrons per kidney), 1 , 2 owing to congenital factors (nephron endowment at birth) and acquired factors (loss of nephrons). Variation in the number of nephrons hinders the assessment of function at the nephron level. In particular, nephron loss may result in a compensatory increase in the single-nephron GFR, . . .