The aim of this study was to examine differences in cycling efficiency between competitive male and female cyclists. Thirteen trained male (mean±SD: 34±8 yr, 74.1±6.0 kg, Maximum Aerobic Power (MAP) 414±40 W, VO 2 max 61.3±5.4 ml· kg-1· min-1) and 13 trained female (34±9 yr, 60.1±5.2 kg, MAP 293±22 W, VO 2 max 48.9±6.1 ml· kg-1· min-1) competitive cyclists completed a cycling test to ascertain their gross efficiency (GE). Leg and lean leg volume of all cyclists was also measured. Calculated GE was significantly higher in female cyclists at 150W (22.5±2.1 vs 19.9±1. 8%; p< 0.01) and 180W (22.3±1.8 vs 20.4±1.5%; p= 0.01). Cadence was not significantly different between the groups (88±6 vs 91±5 rev· min-1). Lean leg volume was significantly lower for female cyclists (4.04±0.5 vs 5.51±0.8 dm 3; p< 0.01) and was inversely related to GE in both groups at 150 and 180W (r=-0.59 and-0.58; p< 0.05). Lean leg volume was shown to account for the differences in GE between the males and females. During an “unloaded” pedalling condition, male cyclists had a significantly higher O 2 cost than female cyclists (1.0±0.1 vs 0.7±0.1 L· min-1; p< 0.01), indicative of a greater non-propulsive cost of cycling. These results suggest that differences in efficiency between trained male and female cyclists can be partly accounted for by sex-specific variation in lean leg volume.