The accurate measurement of renal function in elderly subjects is often required, due to structural and functional changes resulting from age and diseases affecting the kidney. Structurally, with age there is progressive loss of renal mass, particularly in the cortex, leading to a decreased number of glomeruli and an increase in the proportion of sclerotic glomeruli. A decline in the glomerular filtration rate (GFR) has been observed in both cross-sectional and longitudinal studies. However, it has also been noted that about one third of subjects have stable renal function, implying that this decline is not universal, and may reflect the effects of age-associated diseases.