PURPOSE: Even or negative split pacing strategies in long-distance running have been well-established to yield the best performances up to the marathon distance. In addition, women have been found to pace better than men in the marathon. Optimal pacing strategy and sex differences in ultramarathon running are not well-known. The purpose of this study was to determine pacing strategies employed by the fastest finishers in a 100-mile distance run and determine if any sex differences exist. METHODS: Ten years (2010-2019) of finish times from 991 men and 462 women in the Umstead 100-mile endurance run were obtained from publicly available data. The course is a 12.5-mile loop repeated 8 times over relatively flat terrain with 16 splits recorded for each runner at checkpoints (CP) located at miles 5.65 and 12.5. Pace was normalized by expressing the data as %mean running speed (%MRS) and compared by sex and quartile finish. Data were statistically analyzed by one- and two-way ANOVA for repeated measures. RESULTS: Women had a significantly lower %MRS at CPs 2, 3, 4 and 5 as compared to men and had a significantly higher %MRS at the 16th and final CP. Quartile analysis revealed that the fastest runners in the 1st quartile adopted a significantly more even pacing strategy than 2nd, 3rd or 4th quartile finishers and 2nd quartile finishers adopted a significantly more even pacing strategy than 3rd or 4th quartile finishers. Differences in pacing were greatest early in the race with the best performers beginning at 125 + 1.2 %MRS, while the poorest performers began the race at 140 + 1.2 %MRS. While there were few sex differences in 1st, 2nd or 3rd quartile finishers across CPs, in the 4th quartile women adopted a significantly more conservative pace in CPs 1-8, 10 and had a significantly higher %MRS at CPs 14, 15 and 16. CONCLUSIONS: In the 100-mile distance, overall women adopted a more conservative pacing strategy at earlier CPs and finished with a faster %MRS as compared to men. Runners who performed better (finished in higher quartiles) adopted a more even pacing strategy as compared to performers who finished in lower quartiles. The sex difference in pacing was most striking in the 4th quartile with women adopting a more even pacing strategy than men through the first half of the race and finishing at a faster %MRS in the final 3 CPs of the 100-mile distance.