BACKGROUND: Telomeres are nucleoprotein structures that protect the ends of eukaryote chromosomes. Shorter telomere length (TL) is associated with some age-related human disorders, but its relationship with obesity or adiposity parameters remains unclear. OBJECTIVE: The aim of this study was to assess the relationship between TL and changes in adiposity indices after a 5-year nutritional intervention. DESIGN AND SUBJECTS: TL was measured by quantitative real-time PCR in 521 subjects (55-80 years, 55% women). Participants were randomly selected from the PREDIMED-NAVARRA centre after they completed a 5-year intervention programme. Anthropometric parameters were directly measured by trained personnel at baseline and on a yearly basis thereafter. TL at baseline and changes in TL after a 5-year intervention were assessed. RESULTS: Higher baseline TL significantly predicted a greater decrease in body weight (B=-1.09 kg, 95% confidence interval (CI): -2.01 to -0.16), body mass index (BMI) (B=-0.47 kg m(-2), 95% CI: -0.83 to -0.11), waist circumference (B=-1.15 cm, 95% CI: -2.28 to -0.01) and waist to height ratio (B=-0.008, 95% CI: -0.010 to -0.001) in multiple-adjusted models. In addition, changes in TL during the 5-year intervention were inversely associated with changes in the four anthropometric variables. The reduction in adiposity indices during the intervention, associated with increasing TL, was even higher among subjects with the longest telomeres at baseline. Logistic regression analysis showed that the risk of remaining obese after 5 years was lower in those participants who initially had the longest telomeres and increased their TL after intervention (odds ratio=0.27, 95% CI: 0.03-2.03). CONCLUSIONS: Our research suggests that TL is inversely associated with changes in obesity parameters. The assessment of TL can provide further insights for biological pathways leading to adiposity. We show for the first time an improvement of obesity indices when an increase in TL is observed after a 5-year Mediterranean diet intervention.