Using serial metabolic balances, iron absorption was measured in six preterm infants (mean gestational age 29 weeks), and two fullterm small for gestational age (SGA) infants, between day 10 and 70 after birth. They were all fed breast milk. Iron supplements (2.5--13 mg/kg day) were given from day 30. Three preterm infants received blood transfusions for anemia. During the first 30 days of life iron balance was negative in the preterm infants (mean +/- SEM = -0.10 +/- 0.02 mg/kg day) and positive in the full term SGA infants (mean +/- SEM = 0.098 +/- 0.02 mg/kg day). In infants who were not tranfused, absorption of supplementary iron was a linear function of iron intake, and corresponded closely to 34% absorption. An iron intake of 5--6 mg/kg day resulted in the absorption of amounts of iron close to those being laid down in utero. Blood transfusion was followed by a reduction in iron absorption; in two cases it became negative, becoming positive again as the hemoglobin fell below about 12.0 g/100 ml. These data show that a mechanism exists in preterm infants for the control of iron absorption which does not operate at the hemoglobin concentrations that prevail in such infants, unless they are transfused.