Male athletes tend to outperform female athletes in several endurance sports. Maximum cardiac output can be estimated by maximal oxygen consumption ($\dot{V}{O}_{2}max$), and it has been established that men present $\dot{V}{O}_{2}max$ values about 20% higher than women. Although sex differences in $\dot{V}{O}_{2}max$ have already been well studied, few studies have assessed sex differences with regard to muscle oxidative capacity. The aim of this study was to compare aerobic muscle quality, accessed by $\dot{V}{O}_{2}max$ and adjusted by lower limb lean mass, between male and female amateur triathletes. The study also aimed to compare sex differences according to $\dot{V}{O}_2$ submaximal values assessed at ventilatory thresholds. A total of 57 participants (23 women and 34 men), who had been training for Olympic-distance triathlon races, underwent body composition evaluation by dual-energy X-ray absorptiometry and performed a cardiorespiratory maximal test on a treadmill. Male athletes had significantly higher $\dot{V}{O}_{2}max$, both absolutely and when adjusted to body mass. Conversely, when $\dot{V}{O}_{2}max$ was adjusted for lean mass, there was no significant difference between sexes. The same was observed at submaximal exercise intensities. In conclusion, differences in $\dot{V}{O}_{2}max$ adjusted to body mass but not lean mass may explain, at least in part, sex differences in performance in triathlons, marathons, cycling, and other endurance sports.