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To understand the urination pattern and to determine the relationships between urine output and performance of ultramarathon runners.Prospective observational study.The 2005 Soochow University international ultramarathon, in which each athlete ran for 12 hours.All entrants in the 12-hour race were invited to participate in the study.None.Athletes were weighed immediately before and after the race. Urine samples were collected during the race and immediately after the race.There was a trend toward better performance of the group with less urination (LU), although the difference was not statistically significant. Further analysis of hourly running distances between groups showed better performance in the group with LU during the first 11 hours of the competition. Comparison of athletes in 3 levels of running distance (tertiles) showed statistically significant differences between groups in total urine output. The fastest tertile had lower prerace body weight and greater body weight change than the slowest and intermediate tertiles, but the differences were not statistically significant. Linear regression analysis using the stepwise method showed that total urine output and prerace body weight were negatively associated with performance.Runners with LU had better performance during the first 11 hours of the competition. Linear regression analysis showed that total urine output and prerace body weight were negatively associated with performance.