The presence of fetall lung surfactant in amniotic fluid is indicative that fetal lungs are significantly mature for delivery without subsequent respiratory distress. Amniotic fluid, collected from 42 pregnant women less than 24 hours before delivery, was subjected to the determination of L/S ratio and foam test. The results were compared with the incidence of clinical respiratory distress. Of the 27 cases with the L/S ratio above 2.0, none of the newborn infants in this group had clinical respiratory distress. In the high risk group(L/S ratio less than 1.5) clinical respiratory distress occurred in 5 out of 6 infants. All but one with a positive foam test were delivered within 24 hours without clinical respiratory distress, whereas 3 of 13 cases with intermediate foam test results and 2 newborns with negative ones develped clinical respiratory distress. And in 36 of 40 cases with intermediate and positive foam test results, the L/S ratios were above 1.5. These results indicate that the foam-stability test as well as the determination of L/S ratio is a reliable index of fetal lung maturity.