Controversy exists as to whether cortisol acts physiologically to accelerate human fetal lung maturation. Recent data from this laboratory showed that cortisol and corticosterone conjugates (GCS) rise fivefold in amniotic fluid late in pregnancy and correlate well with the palmitic/stearic (P/S) ratio (γ = 0.79). In this study GCS was compared with various tests of fetal maturation. In a series of 38 cases compared with the lecithin/sphingomyelin (L/S) ratio, the GCS and palmitic acid had the highest concurrence (84% agreement), followed by the “shake” test (82%), cortisol (74%), cytology (64%), gestational age determined by ultrasound (63%), and creatinine (47%). The GCS correlate better than cortisol itself, probably because cortisol in amniotic fluid is derived from the chorionic membrane as well as from fetal urine. These observations provide convincing evidence of a physiologic link between fetal lung maturation and alteration in fetal steroid production.