The diagnostic effectiveness of the lecithin:sphingomyelin (L:S) ratio was compared with that of the quantitative amniotic fluid foam stability index (FSI) test in the prenatal evaluation of pregnancies associated with fetuses appropriately grown for gestational age (AGA) and with intrauterine growth retardation (IUGR). In 27 such pregnancies, both the L:S ratio assay and the FSI test were performed on amniotic fluid specimens collected within 72 hours of delivery. Of the 27 low birth weight (LBW) neonates studied, 15 were small for gestational age (SGA) but did not have idiopathic respiratory distress syndrome (IRDS), whereas of 12 AGA neonates, 9 had IRDS. The 15 IUGR cases were characterized by FSI values varying from 0.47 to 0.55, whereas the 12 AGA cases had values varying from 0.42 to 0.48. Thus, the FSI showed excellent differentiation between these 2 entities. The L:S ratio ranged from 1.1 to 3.4 in cases associated with IUGR and from 1.0 to 2.4 in AGA cases, showing a high degree of overlap. The combination of an L:S ratio of less than 1.5 and an FSI value of less than 0.47 consistently identified the fetus who would have IRDS.