Gastrointestinal bleeding has been observed in long-distance runners. We prospectively studied participants of the Eighth Annual Marine Corps Marathon to determine the incidence of gastrointestinal blood loss associated with long-distance running. Of 600 runners contacted, 125 (21%) returned a questionnaire as well as pre- and postmarathon stool specimens. Stool specimens converted from Hemoccult negative to positive in 29/125 (23%) of the participants, indicating that running the marathon was associated with gastrointestinal blood loss (P<0.001). the="" incidence="" of="" this="" conversion="" (negative="" to="" positive)="" was="" significant="" for="" both="" males="">N=68,P<0.001) and="" females="">N=57,P<0.05). gastrointestinal="" bleeding="" appeared="" to="" be="" independent="" of="" age,="" race="" time,="" abdominal="" symptoms,="" and="" the="" recent="" ingestion="" of="" aspirin,="" vitamin="" c,="" or="">