This study was performed to evaluate the effect of iron supplementation on iron-deficiency-related indices, oxidative stress and antioxidative enzyme activity in female marathoners. Fourteen teenage female marathoners participated in the study. Subjects were divided into two groups: mild anemic and control, depending on their hemoglobin (Hb) level. The mild anemic group had significantly lower RBC count and hematocrit (Hct) and Hb levels compared to the control group. The mild anemic group (〈12.5g Hb/dI, n=7) was given iron supplements (60mg Fe/day) for four weeks during the summer training period. RBC count, Hct and Hb levels showed an increasing tendency through iron supplementation, and significant differences in these variables between the anemic and control groups disappeared in the post-period. There was no difference in plasma malondialdehyde (MDA) between the anemic and control groups. However, catalase (CAT) and glutathione peroxidase (GPx) activity were significantly higher in the anemic group. The significant difference in enzyme activity between the groups disappeared in the post-period. In addition, superoxide dismutase activity significantly decreased after iron supplementation. In conclusion, antioxidative enzyme activity was up-regulated in an anemic condition and mild iron supplementation decreased the antioxidant enzyme activity of female marathoners while improving their anemic condition.