Leukocyte telomere length (LTL) a potential marker for biological ageing- a ‘biological clock’. Shorter LTL has been associated with increased morbidity and mortality. While telomeres naturally shorten with cell division, they are also susceptible to damage from the environment, such as reactive oxygen species. At birth, there are no sex differences in LTL, but previous studies of adults have established that telomeres are typically shorter in males as opposed to females. In this study, we investigated the hypotheses that estradiol, total testosterone, and/or body surface area (BSA) mediate sex differences in LTL within our population of early-stage pubescent children.