The aim of this study was to determine if gender differences in muscle mass explain the gender differences in running and cycling sprint performance. Body composition (dual-energy X-ray absorptiometry), and running (30 and 300 m test) and cycling (Wingate test) sprint performance were assessed in 123 men and 32 women. Peak power (PP) output in the Wingate test expressed per kg of lower extremities lean mass (LM) was similar in males and females (50.4 +/- 5.6 and 50.5 +/- 6.2 W kg(-1), P = 0.88). No gender differences were observed in the slope of the linear relation between LM and PP or mean power output (MP). However, when MP was expressed per kg of LM, the males attained a 22% higher value (26.6 +/- 3.4 and 21.9 +/- 3.2 W kg(-1), P < 0.001). The 30 and 300-m running time divided by the relative lean mass of the lower extremities (RLM = LM x 100/body mass) was significantly lower in males than in females. Although, the slope of the linear relationship between RLM and 300-m running time was not significantly different between genders, the males achieved better performance in the 300-m test than the females. The main factor accounting for gender differences in peak and mean power output during cycling is the muscle mass of the lower extremities. Although, the peak power generating capability of the muscle is similar in males and females, muscle mass only partially explains the gender difference in running sprints, even when expressed as a percentage of the whole body mass.