Background : The incidence of anemia in the female athlete is high, and anemia causes not only a decrease in performance, but also various subjective symptoms. It is said that the cause of hemolysis -a type of anemia- is a decrease in antioxidants and an increase in the reactive oxygen species caused by exercise. Purpose : This study investigated the effect of acute exercise on hemolysis and oxidative stress in female athletes. Methods : On the basis of basal body temperature (BBT) data and urinary ovulation tests of the subjects (age 20.5±1.0 yr ), they were divided into two groups : eumenorrheic athletes (Eu, n=12) group, and irregular menstrual athletes (Am, n=9). The subjects performed an acute period of exhausting exercise on a bicycle ergometer. Lactate, hemoglobin, hematocrit, RBC, serum estradiol, haptoglobin, thiobarbituric acid reactive substances (TBARS) were determined in blood samples collected at rest and after exercise. Results : Lactate increased and hemoglobin, hematocrit, RBC, haptoglobin decreased after exercise in both the Eu and Am groups (p<0.05). However, serum TBARS did not show a significant change after exercise in both the Eu and Am groups. Conclusion : These results suggest that hemolysis may have developed, because haptoglobin decreased as a result of acute exercise. However, it is thought that the effect of oxidative stress is small because TBARS were not changed by acute exercise in both Eu and Am groups. In addition, there was no significant correlation between hemolysis and estrogen.