The amount of iron in a 1.0 kg preterm infant at birth is sufficient to synthesise only about 18.0 grams of haemoglobin. Since breast milk contains only 40 microgramsFe/100 ml, anaemia will develop in a premature baby fed breast milk unless supplementary iron is given. Preterm infants fed on breast milk are in negative iron balance averaging -0.24 mg/kg X day for at least thirty days after birth, and it can be estimated that they require an intake of about 0.6 mg/kg X day to compensate for obligatory intestinal iron losses. Insensible skin losses, estimated from measurements in adults, are small--of the order of 0.02 micrograms/kg X day, but losses due to venesection may be considerable since each gram of haemoglobin contains 3.4 mg of iron. Absorption of supplementary iron by preterm infants is a linear function of intake, which suggests immature control of iron absorption. Giving blood transfusions seem to diminish iron absorption but may not prevent it altogether. Giving repeated blood transfusions results in high serum ferritin levels similar to those seen in iron overload--however these levels decline spontaneously with age. Preterm infants who are given repeated transfusions do not require iron supplements until the transfusions cease.