The purpose of this investigation was to evaluate the influence of lean body mass (LBM) and body weight (BW) on the thermoregulatory responses and endurance performance of male and female athletes in warm, humid environments. Ten (5 males, 5 females) healthy, moderately trained athletes with varying physiques performed a self-paced 30-min run on a motorized treadmill in warm (30°C), humid (60% relative humidity) conditions, with the aim of running the greatest distance possible. Males completed one trial, while females completed two trials, one in each of the follicular (Fol) and luteal (Lut) phases of the menstrual cycle in a randomized fashion. There were no significant differences among groups for distance run (males, 5.2 ± 0.4 km; Fol, 4.9 ± 0.1 km; Lut, 4.7 ± 0.1 km). However, following analysis of covariance accounting for LBM and BW, the distances run were significantly different. The adjusted means for distance run after accounting for LBM were 3.4 km for males (P < 0.05), 5.9 km for Fol, and 5.6 km for Lut. Adjusted means accounting for BW resulted in run distances of 6.5 km for males (P < 0.05), 4.2 km for Fol, and 4.0 km for Lut. Thermoregulatory responses such as rectal and skin temperatures were similar among groups. Avenues of heat loss and gain were altered relative to the menstrual cycle phase. The results suggest that one reason for the disparity in performance between male and female athletes over similar race distances might in part be related to unequal body characteristics and in particular to differences in LBM. Am J Phys Anthropol 118:285-291, 2002. Â© 2002 Wiley-Liss, Inc.