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.(ABSTRACT TRUNCATED AT 250 WORDS)