Purpose. The prevalence of exercise-associated hyponatremia (EAH) has mainly been investigated in male endurance athletes. The aim of the present study was to investigate the prevalence of EAH in female 100-km ultra-marathoners and to compare them to male ultra-runners since females are considered more at risk of EAH. Methods. Changes in body mass, hematocrit, [Na+] and [K+] levels in both plasma and urine, plasma volume, urine specific gravity, and the intake of energy, fluids and electrolytes was determined in 24 male and 19 female 100-km ultra-marathoners. Results. Three male (11%) and one female (5%) ultra-marathoners developed asymptomatic EAH. Body mass decreased, while plasma [Na+], plasma [K+] and hematocrit remained stable in either gender. Plasma volume, urine specific gravity and the potassium-to-sodium ratio in urine increased in either gender. In males, fluid intake was related to running speed (r = 0.50, p = 0.0081), but not to the change in body mass, in post-race plasma [Na+], in the change in hematocrit and in the change in plasma volume. Also in males, the change in hematocrit was related to both the change in plasma [Na+] (r = 0.45, p = 0.0187) and the change in the potassium-to-sodium ratio in urine (r = 0.39, p = 0.044). Sodium intake was neither related to post-race plasma [Na+] nor to the change in plasma volume. Conclusions. The prevalence of EAH was not higher in female compared to male 100-km ultra-marathoners. Plasma volume and plasma [Na+] were maintained and not related to fluid intake, most probably due to an activation of the reninangiotensin-aldosterone-system.