We studied the incidences of respiratory distress syndrome (RDS) and of severe RDS and the survival rate in premature newborns who received a complete prenatal course of betamethasone (N = 49) and in those who did not receive the steroid (N = 546). We also studied the same parameters in steroid-untreated prematures who had prolonged rupture of the membranes (greater than or equal to 24 h, N = 91), and in those who did not (N = 448). We found that the incidences of RDS and severe RDS were significantly lower and the survival rate significantly higher in steroid-treated babies than in untreated babies at 28-32 weeks of gestation (29% vs. 58%, P less than 0.01; 21% vs. 46%, P less than 0.03; and 83% vs. 49%, P less than 0.01, respectively). We also found that untreated babies born after prolonged rupture of the membranes at 28-32 weeks had significantly lower incidences of RDS and severe RDS than untreated babies without prolonged rupture (43% vs. 64%, P less than 0.02; 30% vs. 51% P less than 0.02, respectively). Between 28 and 32 weeks, steroid-treated babies did not have significantly lower incidences of RDS and severe RDS than untreated babies born after prolonged rupture of the membranes (29% vs. 43%, P greater than 0.2; 21% vs. 30%, P greater than 0.3, respectively). The data indicate that between 28 and 32 weeks gestation, the protection against RDS that is provided by prenatal treatment with betamethasone is not superior to that provided by prolonged rupture of the membranes alone.