Expression of IL-10 is decreased in lungs of preterm infants. We determined the constitutive and lipopolysaccharide (LPS)-induced IL-10 synthesis by lung inflammatory cells from preterm and term infants and examined their relationship to gestational age and/or incidence of bronchopulmonary dysplasia (BPD). A total of 37 infants; preterm neonates at gestational ages of 23-27 wk (group 1); 28-34 wk (group 2), and four full-term infants with meconium aspiration (group 3) were enrolled. One sample of lung inflammatory cells, obtained during postnatal d 1-3, and another during postnatal d 4-7 were cultured in vitro in presence or absence of 100 mug/mL of LPS. Secreted IL-10 was measured by ELISA. A positive relationship was found between gestational age and LPS-induced, but not constitutive IL-10 production within 1-3 d of life; group 1 on d 1-3 had a significant number of IL-10 nonresponders compared with group 2. All term neonates in group 3 had positive LPS-induced IL-10 response. Thus, in utero maturation of IL-10 gene expression is due to acquisition of inducibility. In contrast, constitutive IL-10 production within d 1-3 of life correlated with, and predicted the incidence of BPD in the highly vulnerable very premature infants.