As expressed in circulating white blood cells (WBCs), telomere length predicts susceptibility to aging-related diseases, even after consideration of chronological age. As such, WBC telomere length is a bio-indicator of human aging. The association between shortened WBC telomere length and lower numbers of circulating endothelial progenitor cells suggests diminished reserve capacity of the hematopoietic stem cell pool, perhaps due to telomere shortening. This might hinder the repair of the vasculature and intensify the atherosclerotic process. From this perspective shortened telomere length might become a bio-determinant in atherosclerosis. The accumulation of WBCs with ultra-short telomeres in the elderly forecasts earlier mortality. However, the direct link between shortened WBC telomere length and replicative senescence of subsets of T lymphocytes, such as CD8+ cells, is not fully established. Knowledge about in vivo telomere dynamics in humans is still evolving. Accordingly, attempts to elongate telomeres in somatic cells, including WBCs, in healthy individuals are premature and should be discouraged at present.