The high affinity Fcgamma-receptor I (FcgammaRI, CD64) is normally expressed only to a very low extent by neutrophils. During bacterial infections, however, neutrophils from adult patients significantly increase their expression of FcgammaRI. Stimulation through FcgammaRI is a highly effective way to improve various aspects of neutrophil function, including phagocytosis. In our study the expression of FcgammaRI on neutrophils from preterm (n = 9) and term (n = 3) newborn infants, children (n = 14), and adults (n = 6) during the early phase of an acute bacterial infection was investigated. Our results showed that neutrophils from newborn infants with bacterial infection expressed FcgammaRI to a significantly higher extent than both noninfected preterm (p < 0.001) and term (p < 0.001) newborn infants and that neutrophils from preterm neonates expressed FcgammaRI to the same extent as neutrophils from term neonates and older infants, children, and adults. No difference in the neutrophil cell surface expression of FcgammaRI during bacterial infections was found among newborn infants, children, and adults. Expression of FcgammaRI probably represents an important mechanism to improve neutrophil phagocytosis as well as other aspects of neutrophil function during bacterial infections, especially in preterm infants. Our study indicates that measurement of cell surface expression of FcgammaRI on neutrophils could be a useful indicator of severe bacterial infections in preterm and term neonates, as well as in older children and adults.