Background: Racial differences in the predilection to salt sensitivity may arise from different renal growth patterns. To test this idea, we monitored age-dependent telomere attrition rate, reflecting largely the replicative history of somatic cells, in the outer renal cortex and the inner renal medulla of African Americans and Caucasians. Methods: Telomere length, determined by the mean length of the terminal restriction fragments (TRF), was measured in specimens from 58 African-American and 63 Caucasian males, ages 1 day to 71 years. Results: In the outer renal cortex, TRF length attrition rate was significantly slower in African Americans (–0.021 ± 0.0064 kb/year) than in Caucasians (–0.060 ± 0.0094 kb/year) (p = 0.0007). In both ethnic groups the TRF length attrition rate was slower in the inner medulla than in the outer renal cortex, but without significant racial differences. Conclusions: The proximal tubule is the most abundant nephron structure in the outer renal cortex. Less proliferative growth of proximal tubular cells in kidneys from African Americans may be one factor explaining the slower age-dependent telomere attrition rate in the outer renal cortex of African Americans than in Caucasians.