cy in sport and physical activity participation accounts for any sex differences in aerobic performance are unknown. The purpose of this study was to 1) identify sex-based differences in aerobic running performance at 1600 m for children aged 6–12 yr and 2) investigate sex-based differences in participation in children and any relationship between participation and sex differences in aerobic performance. Methods We compared 1600-m running velocities (in m·s−1) and participation for 3621 children in the United States in grades 1–6 (ages 6–12 yr) for the years 2007–2014 that were obtained from a publicly available website (runnercard.com). We correlated the female participation with the performance difference between the sexes for each year and grade. We also created a mathematical model to predict the mean velocity () needed to equalize the sex difference in performance if participation was equalized. Results Male children were faster (3.00 ± 0.53 m·s−1) than female children (2.77 ± 0.49 m·s−1) at every grade level, with an average difference of 7.7% (P < 0.001). Participation was lower for female children (46.2%; χ2 = 13.0, P = 0.02) and there was no correlation between female participation and the performance difference between sexes (Pearson’s r = 0.255, P = 0.96). needed to equalize the sex difference in performance if participation was equalized was 4.39 m·s−1, which was greater than the observed female mean plus two standard deviations. Conclusions Male children are faster than female children at running 1600 m at ages 6–12 yr. This sex difference was not a result of lower female participation and suggests that innate physiological sex differences may be responsible....