On 7 August 1954, the world 42 km marathon record holder, Jim Peters, collapsed repeatedly during the final 385 metres of the British Empire and Commonwealth Games marathon held in Vancouver, Canada. It has been assumed that Peters collapsed from heatstroke because he ran too fast and did not drink during the race, which was held in windless, cloudless conditions with a dry-bulb temperature of 28 degrees C. Hospital records made available to us indicate that Peters might not have suffered from exertional heatstroke, which classically produces a rectal temperature > 42 degrees C, cerebral effects and, usually, a fatal outcome without vigorous active cooling. Although Peters was unconscious on admission to hospital approximately 60 minutes after he was removed from the race, his rectal temperature was 39.4 degrees C and he recovered fully, even though he was managed conservatively and not actively cooled. We propose that Peters' collapse was more likely due to a combination of hyperthermia-induced fatigue which caused him to stop running; exercise-associated postural hypotension as a result of a low peripheral vascular resistance immediately he stopped running; and combined cerebral effects of hyperthermia, hypertonic hypernatraemia associated with dehydration, and perhaps undiagnosed hypoglycaemia. But none of these conditions should cause prolonged unconsciousness, raising the possibility that Peters might have suffered from a transient encephalopathy, the exact nature of which is not understood.