1 Human Biology in press Dear Editor We write to point out that digit ratio may be a useful childhood biomarker for endurance running in adults. We have considered this question in a cohort of Jamaican children who have been studied from 1996 to the present day (Trivers et al, 1999). Digit ratio (2D:4D: the relative lengths of 2nd and 4th digits) is a negative correlate of prenatal testosterone (Manning et al, 2007). Prenatal testosterone (PT) has organising effects on many traits, and one such is performance in sport. Low 2D:4D (high PT) is linked to endurance sports but it is unclear whether 2D:4D measured in children and teenagers is predictive of their performance when they are adults (Manning et al, 2007). The purpose of this letter is to consider this question in a long-term study (The Jamaican Symmetry Project: JSP) of participants from a rural Jamaican population (Trivers et al, 1999). The JSP consists of a cohort of 288 children (155 boys), that were first measured in 1996 when participants were 5 to 11 years. The Project focuses on developmental stability, but many other traits have been included. 2D:4D was measured in 1996 from hand X-rays and also from hand photocopies in 2002, and in 2010 for the current study. These measurements afforded us the opportunity to consider a longitudinal study of childhood, teenage and adult 2D:4D as predictors of endurance-related adult performance in sport. We examined the participant's performance in two races (90m and I80m) and focussed on the participants time for the 180m relative to their time for the 90m. We considered the hypothesis that low 2D:4D is linked to endurance and made the following predictions, (i) participants with low 2D:4D would run faster in the 180m than EFTA00678566