Following planned termination of pregnancy, infants of diabetic mothers have shown a continued vulnerability to the respiratory distress syndrome (RDS), suggesting false positive results on fetal lung maturity testing. We studied the adequacy of the standard one-dimensional chromatographic lecithin/sphingomyelin (L/S) ratio for predicting RDS in 56 diabetic and 50 nondiabetic pregnancies. An exceptionally low incidence of RDS was noted for the study (0.95%) as well as for diabetic pregnancies alone (1.85%). The clinical results were further compared to a new method for determining L/S ratios and identifying other pulmonary phospholipids by two-dimensional chromatography. Values for two-dimensional L/S ratios were significantly lower from 36 through 38 weeks' gestation (P less than 0.01), suggesting improved accuracy and a lessened risk for false positive results in preterm deliveries. Positive identification of amniotic fluid phosphatidylglycerol (PG) was related to increasing L/S ratios (P = 0.001), but only 50% of nondiabetic and 16.7% of diabetic amniotic fluid samples contained PG when the L/S was 2:1 or greater (P = 0.02). The identification of PG serves to confirm fetal lung maturity, but the L/S ratio remains the best available indicator of fetal lung maturity for all high-risk pregnancies.