BACKGROUND: In spite of the beneficial effect of iron supplementation in iron-deficient pregnant women, iron supplementation may not be needed for women who are iron replete or not anemic. Moreover, the theoretical possibility of adverse effects, such as oxidative damage,from administration of iron supplements during pregnancy has been raised. OBJECTIVE: To determine the effect of prophylactic iron supplementation on iron status and birth outcomes among nonanemic pregnant women. METHODS. A randomized, triple-blind clinical trial was conducted. One hundred forty-eight nonanemic pregnant women with hemoglobin > 110 g/L, serum ferritin > 12 microg/L, and gestational age < 20 weeks were randomly assigned to receive either ferrous sulfate (60 mg elemental iron) (n = 70) or placebo (n = 78) until delivery. Hemoglobin concentration and serum ferritin were measured by electronic counter and radioimmunoassay, respectively. Data were analyzed by independent t-tests, ANCOVA, and repeated measures. RESULTS: At delivery, the incidence of iron deficiency was significantly lower in the iron than in the placebo group. There were no significant differences between the two groups in maternal hemoglobin and ferritin concentrations at delivery or in the infant's birthweight, birth length, or length of gestation. CONCLUSIONS: Iron supplementation during pregnancy in nonanemic women with low serum ferritin may have benefits beyond the prevention of iron-deficiency anemia.