Delineation of neutrophilic pool sizes and kinetics during the steady state and during various perturbations has improved the clinical approach to adult patients with abnormalities of blood neutrophil concentration. Perhaps a better understanding of neutrophil kinetics in the fetus and neonate might similarly permit a less empiric approach to neutropenia and neutrophilia in newborn infants. The relatively few studies that have been performed suggest that substantial differences in pool sizes and kinetics exist between neonates and adults, with even greater differences between extremely preterm infants and those delivered at term.