Leukocyte telomere length (LTL) is ostensibly a bio-indicator of human aging. Here we report that African Americans have longer LTL than whites. We studied cross-sectionally 2453 individuals from the National Heart, Lung, and Blood Institute (NHLBI) Family Heart Study (age = 30-93 years) and the Bogalusa Heart Study (age = 19-37 years), comprising 1742 whites and 711 African Americans. We measured LTL by Southern blots of the terminal restriction fragments length. In 234 participants, telomere repeats were also measured by quantitative polymerase chain reaction (qPCR). Adjusted for age and body mass index (BMI), the respective leukocyte telomere lengths (mean +/- SEM) were considerably longer in African Americans than in whites both in the Family Heart Study (7.004 +/- 0.033 kb vs. 6.735 +/- 0.024 kb, p < 0.0001) and the Bogalusa Heart Study (7.923 +/- 0.063 kb vs. 7.296 +/- 0.039 kb, p < 0.0001). We confirmed the racial effect on LTL by qPCR (3.038 +/- 0.565 T/S units for African Americans vs. 2.714 +/- 0.487 T/S units for whites, p < 0.001). Cross-sectionally, sex- and BMI-adjusted LTL became shorter with age (range 19-93 years) at a steeper slope in African Americans than in whites (0.029 kb year(-1) vs. 0.020 kb year(-1), respectively, p = 0.0001). We suggest that racial difference in LTL arises from a host of interacting biological factors, including replication rates of hematopoietic stem cells.