We measured RBC count, blood hemoglobin concentration, and related hematologic factors in 12 male marathon runners during a 20-day, 312-mile road race. The RBC and hemoglobin levels decreased significantly, from 5.17 million per cubic millimeter and 16.0 g/dL, respectively, to 4.36 million per cubic millimeter and 13.4 g/dL. Although the runners became marginally anemic during the race, their running speeds were not significantly changed. There were no abnormal clinical signs other than rare instances of hematuria. This study indicates that a sudden increase in long-distance running mileage above the regular training level may cause "sports anemia," a functional pseudoanemia that, in mild cases, does not impair endurance performance.