Ferritin concentrations in cord blood were determined in 22 normal term and 32 preterm infants (birth weights 600-2000 g). Eight of the preterms were SGA infants. AGA preterm infants had significantly lower concentrations than term infants, and the SGA preterm newborn had even lower levels. Plasma ferritin in cord blood of the term and AGA preterm infants correlated positively with plasma iron and transferrin saturations, but not with the transferrin level, while plasma iron and transferrin concentrations correlated positively. In a longitudinal study, 17 AGA preterm infants (birth weights 850-1500 g) were followed during the early anaemia of prematurity. Iron was supplemented from 4 weeks of age. Plasma ferritin rose rapidly during the first days after birth, peak levels being reached at 1-4 weeks. Thereafter linear falls (semilog) occurred with similar slopes in different infants. Transferrin concentrations showed a slow progressive increase from 0-8 weeks. Plasma ferritin, after reaching the peak value, correlated negatively with weight gain. No infant had low ferritin values indicating iron deficiency during the early anaemia.