The histology and function of the kidney deteriorates with age and age-related diseases, but the mechanisms involved in renal aging are not known. In vitro studies suggest that telomere shortening is important in replicative senescence, and is accelerated by stresses that increase replication. This study explored the relationship between age and telomere length in surgical samples from 24 human kidneys, which were either histologically normal (17) or displayed histologic abnormalities (7). Telomere loss was assessed by two independent methods: Southern blotting of terminal restriction fragments (TRF) and slot blotting using telomere-specific probes. The results of these methods correlated with each other. The mean TRF length determined by Southern blotting in cortex was about 12 kb pairs (kbp) in infancy and was shorter in older kidneys. The slope of the regression line was about 0.029 kbp (0.24%, P = 0.023) per year. Telomere DNA loss in cortex by the slot blot method was 0.25% per year (P = 0.011). By both methods, the telomere loss in medulla was not significant and was less than in cortex. Comparisons of TRF length from 20 paired samples from cortex and medulla showed that TRF was greater in cortex than medulla, with the differences being greater in young kidneys and lessening with age due to telomere loss in cortex. These findings indicate that telomeres shorten in an age-dependent manner in the kidney, either due to developmental factors or aging, particularly in renal cortex.