Urogastrone was measured by radioimmunoassay in amniotic fluid obtained from 186 complicated pregnancies at 22 to 40 weeks gestation. Amniocentesis was performed for a variety of indications to obtain information about fetal lung maturity or bilirubin levels before induction of labour or caesarean section in various obstetric conditions. In 114 specimens lung phospholipids extracted from amniotic fluid were also assayed using two-dimensional thin layer chromatography. Urogastrone concentrations became measurable at approximately 30 weeks gestation and thereafter there was a 10-fold rise in concentrations between 30 and 40 weeks gestation. This increase in urogastrone concentration was positively correlated with a rise in phosphatidylcholine and phosphatidylglycerol concentrations and the phosphatidylcholine (lecithin)/sphingomyelin ratio (L/S). These results are compatible with a role for urogastrone in human fetal lung maturation.