Telomeres—the end segments of chromosomes—undergo shortening with successive cycles of cell division, contributing to cellular ageing. Honig et al. studied telomere length in blood samples from 1,983 individuals aged 65 years and over, who were followed up for a median of 9.3 years. After correcting for baseline age and other confounding factors, shorter telomere length was associated with younger age at death and, in women, with increased risk of dementia. Leukocyte telomere length could, therefore, represent a marker of biological ageing.