Iron deficiency, with or without anemia, occurs commonly in long-distance runners, but the cause is unknown. The recent development of a simple quantitative assay for fecal hemoglobin, HemoQuant , allowed us to study whether gastrointestinal bleeding occurs in runners. Blood and stool samples were collected from 24 runners before and after a race of 10 to 42.2 km and from age- and sex-matched, nonrunning controls. The mean blood hemoglobin level and hematocrit were significantly lower in runners than in controls. Serum ferritin levels were below normal in 4 runners but in no controls. Fecal hemoglobin levels increased in 20 of 24 runners (p less than 0.01) after a race. Mean fecal hemoglobin level was 1.08 mg/g (range, 0.11 to 2.36) in controls and 0.99 mg/g (0.18 to 2.41) in runners before a race, but peaked at 3.96 mg/g (0.37 to 43.20) after a race. Competitive long-distance running induces gastrointestinal blood loss and may contribute to iron deficiency.