We analyzed 331 pregnancies where clear, uncontaminated amniotic fluid was obtained within 72 hours of delivery from both normal and complicated gestations. There were 43 cases of the neonatal idiopathic respiratory distress syndrome (IRDS), 42 of which were associated with foam stability index (FSI) values of less than 0.47. In 276 of the 323 cases sufficient amniotic fluid was available to permit concurrent performance of the lecithin/sphingomyelin ratio (L/S) assay. Thirty-five cases of IRDS occurred in this group, 8 of which were associated with L/S ratio values greater than 2.0. The risk of IRDS occurrence was calculated for three groups of FSI values (shown in parentheses below). They were as follows: group I--FSI values less than or equal to 0.43 (73%); group II--FSI values of 0.44 to 0.46 (29.2%); group III--FSI values greater than or equal to 0.47 (0.35%). Comparable evaluations for the L/S ratio procedure showed the FSI test to be significantly more specific and sensitive than the L/S ratio assay. The improved performance of the FSI test was most marked in pregnancies complicated by insulin-dependent diabetes mellitus and intrauterine growth retardation. The low cost, ease and rapidity of the FSI test, as well as the fact that it does not require sophisticated technical equipment or expertise, makes this sensitive and specific assay a method of choice for the prenatal evaluation of fetal pulmonary maturity.