PURPOSE: The sex difference in ultra-endurance performance has been investigated in swimmers, runners and triathletes, but not in cyclists. The purpose of this study was to examine the sex difference in the longest ultra-cycling race in the world, the 'Race Across America' (RAAM). METHODS: Cycling speed of female and male finishers in the 'RAAM' between 1982 and 2012 was compared. RESULTS: A total of 452 athletes including 404 men (89.4%) and 48 women (10.6%) finished. Mean cycling speed was 19.4±2.0 km/h for men and 17.5±2.0 km/h for women. Men were riding 1.9±2.0 km/h (10.9%) faster than women. The fastest cycling speed ever was 24.77 km/h for men and 21.27 km/h for women with a sex difference of 14.2%. Between 1982 and 2012, cycling speed was 22.7±1.1 km/h for the annual fastest men and 18.4±1.6 km/h for the annual fastest women with an unchanged sex difference of 19.4±7.3% (P>0.05). For the annual top three men, cycling speed was 21.8±0.9 km/h with no change across years (P>0.05). The annual top three women achieved a cycling speed of 16.6±1.0 km/h with no change over time (P>0.05). The sex difference of 24.6±3.0% showed no change across years (P>0.05). CONCLUSIONS: In the last 30 years, men crossed America faster than women and it seems unlikely that women will overtop men in the near future in the 'RAAM'. However, the sex difference was only 14-15% among top competitors. Future studies need to analyze anthropometric, psychological and physiological characteristics of successful female and male ultra-cyclists.