The authors studied the correlation between optical density at 650 nm (OD650) in paired samples of amniotic fluid and newborn gastric aspirate obtained from 50 pregnant women who underwent cesarean delivery and from their respective newborns. There was a good correlation between both samples (r = 0.92), which demonstrates that OD650 is a reliable method for studying neonatal pulmonary maturity, especially in those high-risk cases in which no prenatal study is available. None of the infants with mature readings (OD650 > or = 0.10) and 6 out of 13 infants with immature readings (OD650 < 0.10) developed RDS.