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The belief that dehydration poses significant health risks for endurance athletes, especially marathon and ultramarathon runners, stems from the classical 1969-study of Wyndham and Strydom entitled “The Danger of an Inadequate Water Intake During Marathon Running.” The subsequent influence of the paper relates more to its incorrect title than to its scientific content. For the authors did not study nor did they identify any dangers resulting from an inadequate water intake during marathon running. In fact, the most dehydrated runners in their studies were also the most successful, as they won the competitive races that were studied. The positive result of the study was to influence international rule changes to allow increased fluid intake during competitive running races. The less desirable effect was to induce a dogmatic zeal among sports medicine practitioners who began to extol the dangers of dehydration during exercise. The (il)logic spurring this zeal seems to have been the conclusion that progressive dehydration during exercise will cause heatstroke, which is the most important cause of collapse during exercise. Hence, (i) heatstroke during running can only be avoided if dehydration is prevented, and (ii) all persons who collapse in association with exercise will have a heat disorder, which must be treated with intravenous fluid therapy. This article reviews the evidence which shows (i) that the levels of dehydration commonly measured in endurance athletes (1–4% of body weight) cause measurable physiological change and impair exercise performance in the heat but are not associated with health risks such as heatstroke or acute renal failure; (ii) that most athletes collapse after exercise and have rectal temperatures that are no higher than values measured in control runners; and (iii) that no study has yet shown that collapsed runners are more dehydrated than controls. It is proposed that postural hypotension, unrelated to levels of dehydration, is the most common cause of the exercise-associated collapse that occurs after the athlete stops exercising. Clinicians treating athletes with exercise-associated collapse should initiate treatment only after a thorough clinical evaluation has produced rationale differential diagnoses. Intravenous fluid therapy should be reserved only for those patients with clear clinical evidence for dehydration which contributes to their clinical presentation.