Anemia in runners is common but its origin is unknown. The present study reports on frequency and origin of gastrointestinal blood loss in cross-country skiers and runners. 41 participants in the Engadin Ski Marathon were checked by questionnaire and occult blood test. 8 (19%) had diarrhea or abdominal pain during or immediately after skiing and 3 (7%) had hemoccult-positive stools. In addition, the blood flow of the superior mesenteric artery was measured by duplex scanning in two trained runners after standardized exercise. While the mesenteric blood flow in the asymptomatic runner changed only insignificantly there was an impressive decrease in the second runner, who had been treated for anemia, down to 20% and 40% (30 and 90 min respectively) after exercise. It is concluded that the occurrence of gastrointestinal blood loss in cross-country skiing, and the significant decrease in mesenteric blood flow in a symptomatic runner, indicate a close and possible causal relationship between mesenteric ischemia and "jogging anemia".