Iron nutrition was measured in 84 low-birth-weight infants. At birth, they were assigned to three groups: preterm infants appropriate for gestational age (n= 29); preterm infants small for gestational age (n= 17); and full-term infants* small for gestational age (n= 38). A sub-sample of infants was supplemented with iron 3 mg/kg from two to four months of age. At birth, preterm appropriate-for-gestational-age infants had a lower hemoglobin concentration than full-term small-for-gestational-age infants (p < 0.01) and a higher serum ferritin than preterm small-for-gestational-age infants (p < 0.05). In the non-supplemented group, full-term small-for-gestational-age infants had significantly higher hemoglobin concentrations at four months of age. At this age, iron-supplemented preterm infants appropriate or small for gestational age had significantly higher hemoglobin levels than non-supplemented subjects, while iron supplementation did not have an effect on final hemoglobin concentration in full-term small-for-gestational-age infants. We conclude that preterm infants, irrespective of their adequacy for gestational age, show evidence of iron deficiency before four months of age. Full-term infants do not develop iron deficiency up to this age.