This paper summarizes the main findings of 3 publications of our group [2-4] examining fluid balance at high altitude. Of 57 mountaineers ascending from 1170 m to 4559 m within 22 to 77 hours, 24 developed acute mountain sickness (AMS) and 16 developed high altitude pulmonary edema (HAPE). In 14 cases HAPE was preceded by symptoms of AMS. Independently of the amount of fluid intake, which varied from 2 to 4 l/24 h in these studies, subjects developing AMS showed decreased diuresis and natriuresis compared to healthy controls with similar fluid intake. Higher fluid intake resulted in greater urine output but did not prevent AMS. Higher plasma levels of aldosterone at rest and greater exercise-induced rises of plasma aldosterone and vasopressine may explain the increased water and salt retention in subjects with AMS. Whether these hormonal changes are secondary to a more severe hypoxemic stress or present a primary cause of AMS remains to be determined.