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Describe risk factors for lower post-race [Na+] and exercise-associated hyponatremia (EAH) (serum [Na+] <135 mmol/L) during marathon running.Prospective observational study.Houston Marathon 2000-2004.Ninety-six runners from EAH research projects.Observational.Pre-race and post-race measurements: serum [Na+], weight, and fluid ingestion.Twenty-one runners (22%) met criteria for EAH, and 87% of subjects had lower post-race [Na+] compared to pre-race [Na+]. Lower post-race [Na+] and larger [Na+] decrease were related to lower pre-race [Na+], less weight loss during the race, and more fluid cups consumed. Increased fluid consumed correlated with slower finish time, male gender, and warmer temperature. Less weight loss correlated with lower pre-race weight, more fluid consumed, and slower finish time. Losing less than 0.75 kg increased the risk of becoming hyponatremic 7 fold (RR = 7.0; CI 1.8 to 26.6) compared to those who lost more than 0.75 kg. Women consumed fluid at a significantly lower rate than men (P = 0.04). Estimated mean fluid balance for women was positive and significantly higher than men's negative fluid balance (P < 0.0001). Fluid balance became more positive as finish time increased and pre-race weight decreased.Lighter and slower runners have more positive fluid balance. Losing >0.75 kg of body weight during a marathon is advisable in order to decrease the risk of EAH. Runners should measure their sweat rate and monitor weight changes as part of their fluid consumption plan.