While recent studies suggest that cortisol plays a role in lung maturation, conflicting reports have recently appeared relating cortisol in amniotic fluid to indices of lung maturation. The variation in the levels and differences in correlation appear to be due to differences in methodology, principally with respect to cross-reactivity in the radioimmunoassays with the conjugates of cortisol and corticosterone. We have found that the glucocorticoid conjugates correlate better with lung maturation (rs = 0.79) than does cortisol itself (rs = 0.58). This is probably because the conjugates are derived exclusively from the fetal compartment whereas cortisol is also produced by the chorionic membrane. In anencephaly, conjugate levels were very low while cortisol levels were relatively normal. Conjugate levels (mainly sulfates) appear promising as an adjunct to tests of fetal maturation.