Two groups of long-distance runners were investigated for the effect of marathon running on the gastrointestinal mucosa. In one group gastric erosions with bleeding were found in five of nine subjects, mostly localized to the corpus region. The relative gastric blood flow measured by endoscopic laser Doppler flowmetry was slightly decreased in the cardia region (from 7.0 to 5.8; p less than 0.05) but unchanged in the other parts of the stomach, including the erosive lesions. In another group (n = 8) all the subjects showed a substantial increase in the urinary excretion of 51Cr-labeled ethylenediaminetetraacetic acid after oral intake, which indicates an increase in the intestinal permeability. There is reason to suggest that long-distance running affects the integrity of the gastric and the intestinal mucosa.