Prevention of iron deficiency in low-birth-weight infants requires iron supplementation before neonatal iron stores are exhausted. In order to accurately determine when this depletion occurs, we measured the hemoglobin, mean corpuscular volume, serum iron/iron-binding capacity, and serum ferritin in 117 low-birth-weight infants (1,000 to 2,000 gm) from 0.5 until 6 months of age. All infants received banked breast milk in the hospital and breast milk or cow milk formula later; those with odd birth dates received 2 mg iron as ferrous sulfate/kg/day starting at 0.5 months; those with even birth dates received no additional iron unless they developed anemia. The results indicate that low-birth-weight infants who receive no supplemental iron may develop iron deficiency by three months of age and that a dose of iron of 2 mg/kg/day started at two weeks of age prevents iron deficiency without providing excess.