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To assess (1) the incidence of dysnatremia in collapsed runners presenting to the medical tent of the 89-km Comrades Marathon and whether dysnatremia influences time to discharge, and (2) whether intravenous fluids could restore serum sodium concentration ([Na+]) to 140 mM faster than could the administration of oral fluids.Prospective randomized controlled trial.2005 Comrades Marathon.One hundred thirty-three collapsed runners and 31 control-group runners.Collapsed runners presenting to the medical tent at the finish of the 2005 Comrades Marathon were randomized into an intravenous or oral fluid administration group, with the type and amount of fluid administered dictated by initial [Na+].Time to discharge, serum [Na+].Forty-five percent of collapsed runners were hypernatremic, 2% were hyponatremic, and 53% were normonatremic. Normonatremic runners spent significantly less time in the medical tent (80 ± 31 minutes) compared with hypernatremic (102 ± 36 minutes) and hyponatremic (146 ± 122 minutes) runners. Intravenous fluid therapy produced larger but nonsignificant reductions in [Na+] than oral therapy (−2.1 ± 3.1 versus −0.7 ± 1.8 mM); however, 45% of runners assigned to the oral fluid group could not tolerate oral rehydration.A slight majority of collapsed runners were normonatremic and spent significantly less time in the medical tent compared with hyper- and hyponatremic athletes. Initial rates of correction of hypernatremia were similar with intravenous and oral hypotonic fluid therapy. Clinicians should be advised that intravenous fluid resuscitation may best benefit hypernatremic collapsed runners who are intolerant to oral fluid ingestion.