Gastrointestinal bleeding has been proposed to be a contributing factor in the development of 'runner's anaemia'. To study the incidence of gastrointestinal bleeding, 2-3 prerace and 2-3 postrace faecal samples from 63 marathon runners were tested for the presence of blood. Five of the participants had faecal blood before but not after the race. A reasonable explanation for the bleeding was found for four. Eight runners (13%) had positive tests for faecal blood after the competition. Clinical investigation disclosed no gastrointestinal disease. Postrace haematuria was discovered in another 13% of the runners. None of the runners observed overt gastrointestinal bleeding or haematuria, nor did anyone develop anaemia. Gastrointestinal disturbances related to running were reported by 54%. It is concluded that gastrointestinal complaints and gastrointestinal bleeding are prevalent among marathon runners.