To determine whether normal physiologic changes associated with hormone fluctuations over the menstrual cycle affect concentrations of iron-status indicators, we examined data from 1712 women aged 18-44 y from the Second National Health and Nutrition Examination Survey (NHANES II) after adjusting for potential confounders. Adjusted mean values of hemoglobin (Hb), transferrin saturation (TS), and serum ferritin (SF) were lowest for women whose blood was drawn during menses and highest for women examined in luteal or late luteal phase of the menstrual cycle (Hb = 130 vs 133 g/L; TS = 21.2% vs 24.8%, P < 0.01 for both; and SF = 17.2 vs 24.0 micrograms/L, P < 0.05). The prevalence estimate of impaired iron status was significantly higher for women whose blood was drawn during the menstrual phase than for women whose blood was drawn during the luteal and late luteal phases. Our findings suggest that the phases of the menstrual cycle affect the concentration or values of iron-status indicators. These cyclic variations in indicators of iron status are a potential source of error when iron status is assessed in large population surveys that include women of reproductive age.