The relation of the concentrations of prolactin and cortisol in cord plasma to the incidence of respiratory distress syndrome (RDS) due to hyaline membrane disease was investigated in 191 neonates of which 121 were born at gestational ages of 36 weeks or less. Of these 121 infants, 18 were born at gestational ages of 25-29 weeks, 35 at 29.5-33 weeks, and 68 at 33.5-36 weeks. Thirty-six of the 53 babies that were born at gestational ages of 33 weeks or less developed RDS. Nine of the 68 infants that were born at gestational ages of 33.5-36 weeks developed RDS. Three of 70 babies that were born after the 36th gestational week developed RDS. The incidence of RDS among babies having cord plasma prolactin levels less than 200 ng/nil was significantly greater than that in those having prolactin concentrations greater than 200 ng/ml. The plasma prolactin levels were significantly correlated with the weights of the newborn infants as well as with their gestational ages. In contrast to the findings on prolactin levels and the incidence of RDS, there appeared to be no correlation between cord plasma cortisol levels and the incidence of RDS. On the basis of these results, it is suggested that fetal lung maturation and augmented surfactant formation are functions of the fetal prolactin concentration. © 1978 The American College of Obstetricians and Gynecologists.