The foam stability index (FSI) test for amniotic fluid is a quantitative test that uses ethanol as the antifoaming reagent; it presents values in terms of the highest ethanol volume fraction that will permit stable foam to occur. In this study the authors report on FSI tests results in terms of neonatal outcome in a total of 208 pregnancies in which fluids were obtained within 3 days of delivery. The authors noted a total of 27 cases of hyaline membrane disease (HMD) in their population. All 27 had FSI values of 0.47 or less, whereas 119 of the 181 patients without HMD had FSI values of 0.48 or above. The risk of HMD's occurring was calculated for 4 groups of FSI values. They were as follows: 1) FSI values less than or equal to 0.43 (62%); 2) FSI values of 0.44 and 0.45 (23%); 3) FSI values of 0.46 and 0.47% (14%); and 4) FSI values greater than 0.48 (0%). Comparable evaluations of the lecithin:sphingomyelin (L:S) ratio procedure performed on the same specimens revealed no apparently improved diagnostic reliability. In addition, in several cases of intrauterine growth retardation with normal neonatal respiratory status in which the L:S ratios were less than 1.5, the FSI test correctly predicted fetal lung maturity (FSI greater than or equal to 0.48). The authors conclude that the FSI test has sufficient diagnostic reliability to be considered as a sensitive and specific assay of fetal pulmonary maturation.