Objective: To compare outcomes among neonates delivered after documentation of fetal lung maturity before 39 weeks and those delivered at 39 or 40 weeks. Methods: This was a retrospective cohort study of women with singleton pregnancy delivered at 36 0/7 to 38 6/7 weeks after positive fetal lung maturity testing (based on amniotic fluid lecithin to sphingomyelin ratio) or at 39 0/7 to 40 6/7 weeks (without maturity testing) at our center from 1999 to 2008. Women with fetuses with major congenital anomalies, cord prolapse, nonreassuring antepartum testing, placental abruption, or oligohydramnios were excluded. A primary composite neonatal outcome included death, adverse respiratory outcomes, hypoglycemia, treated hyperbilirubinemia, generalized seizures, necrotizing enterocolitis, hypoxic ischemic encephalopathy, periventricular leukomalacia, and suspected or proven sepsis. Results: There were 459 neonates delivered at 36 to 38 weeks and 13,339 delivered at 39 to 40 weeks; mean birth weight was 3,107±548 g and 3,362±439 g, respectively. The risk of the composite adverse neonatal outcome was 6.1% for the 36- to 38-week group compared with 2.5% for the 39- to 40-week group (relative risk 2.4; confidence interval [CI] 1.7-3.5). After multivariable adjustment, early delivery remained significantly associated with an increased risk of the composite outcome (adjusted odds ratio [OR]1.7; CI 1.1-2.6) as well as several individual outcomes, including respiratory distress syndrome (adjusted OR 7.6; CI 2.2-26.6), treated hyperbilirubinemia (adjusted OR 11.2; CI 3.6-34), and hypoglycemia (adjusted OR 5.8; CI 2.4-14.3). Conclusion: Neonates delivered at 36 to 38 weeks after confirmed fetal lung maturity are at higher risk of adverse outcomes than those delivered at 39 to 40 weeks. © 2010 by The American College of Obstetricians and Gynecologists. Published by Lippincott Williams & Wilkins.