We determined prospectively if the stable microbubble (SM) test on gastric aspirate obtained at birth was as useful as that on amniotic fluid in predicting respiratory distress syndrome (RDS). One hundred and five paired samples of amniotic fluid obtained at delivery from 105 consecutive women with gestation of 35 weeks or less and gastric aspirates from their neonates obtained within 30 min of birth were studied. The SM test with the predefined cut-off value of less than 5 bubbles/mm2 for amniotic fluid and less than 10 bubbles/mm2 for gastric aspirate signified the risk of RDS with the positive predictive value of 100% and 96% and with the negative predictive value of 91% and 84%, respectively. We conclude that the SM test on both amniotic fluid and gastric aspirate obtained at birth is a rapid (< 10 min), simple and reliable procedure for predicting neonates who will develop RDS. It may be used as a bedside test to define a population of neonates with surfactant deficiency in clinical trials of prophylactic surfactant therapy.