We studied the effect of iron supplementation on the iron status of mothers and on biochemical iron status and clinical and anthropometric measures in their infants. The subjects were 197 pregnant women selected at 28 wk +/- 21 d of gestation at a mother-and-child health center in Niamey, Niger. Ninety-nine women received 100 mg elemental Fe/d throughout the remainder of their pregnancies and 98 received placebo. The prevalence of anemia and iron deficiency decreased markedly during the last trimester of pregnancy in the iron-supplemented group but remained constant in the placebo group. Three months after delivery, the prevalence of anemia was significantly higher in the placebo group. At delivery, there were no differences between the two groups in cord blood iron variables. Three months after delivery, serum ferritin concentrations were significantly higher in infants of women in the iron-supplemented group. Mean length and Apgar scores were significantly higher in infants with mothers in the iron group than in those with mothers in the placebo group.