Objective: the objective of the study was to assess the efficacy of maternal betamethasone for improving preterm lung function in the presence of inflammation induced by amniotic fluid Ureaplasma colonization.Study DesignEwes bearing single fetuses were randomized to receive an intraamniotic injection of Ureaplasma parvum (serovar 6; 2 × 107 colony-forming units) or vehicle at 86 ± 2 days of pregnancy (mean ± SD: term is 150 days), followed by maternal intramuscular betamethasone (0.5 mg/kg) or saline, either 2 or 7 days before delivery of lambs at 123 ± 1 d.ResultsAmniotic fluid interleukin-8 was elevated by ureaplasmas (P = .049) but unaffected by betamethasone. Lung inflammation induced by ureaplasmas was not affected by betamethasone. Lung compliance was increased by Ureaplasma colonization (P = .009) and betamethasone (P = .042), and effects were additive. Lung surfactant was increased by Ureaplasma colonization (P <.001) and betamethasone 7 days (P = .001), but not 2 days, before delivery.ConclusionInflammation improves preterm lung function because of increases in surfactant. Antenatal corticosteroids further augment lung function through an apparently independent mechanism.