Leukemoid reaction in low-birth-weight (LBW) infants is a rare, recently documented phenomenon, implicated in the sequence of multiorgan inflammatory diseases of preterm infants. The aim of the present paper is to establish whether a neonatal leukemoid reaction is related to bronchopulmonary dysplasia (BPD) development in LBW infants. The design was a case-controlled, retrospective study of all premature infants (born </=31 weeks' gestation) admitted to the neonatal intensive care unit (NICU) over a period of 3 years, from January 1998 to December 2000. The infants who developed BPD formed the study group, while the remainder without pulmonary sequelae, matched for gestational age formed the control group. Leukemoid reaction was considered a white blood cell (WBC) count >40,000/mm 3. The relation between BPD occurrence and WBC counts was studied by Bayesian analysis, dividing WBC counts in 5 progressive bands of 10,000 WBC/mm 3, starting from <10,000 to >40,000/mm 3. Five of 50 BPD infants studied demonstrated WBC counts >40,000/mm 3, with an incidence of 10%, while no control preterm infants presented neonatal leukemoid reaction; the estimated number difference is statistically significant ( p <0.001). There was no other significant association demonstrated between maternal or neonatal variables and leukemoid reaction, including chorioamnionitis, sepsis, and the use of antenatal steroids. Our findings provide further data for the identification of prematures exposed to pro-inflammatory cytokines in utero; the injury responsible for BPD in a subset of prematures may begin with a transient leukemoid reaction.