One hundred forty-six pregnant women were enrolled in a prospective double-blind study to assess the effectiveness and side-effects of antenatal administration of betamethasone in the prevention of respiratory distress syndrome (RDS) in potentially premature infants. On admission to the study, the women were given, at random, either 12 mg of betamethasone or placebo. The same dose was repeated 24 hours later and then weekly up to 34 weeks of gestation. Gestational age of the infants ranged from 25 to 34 weeks, and birth weights ranged between 730 and 2,650 gm. Statistically significant differences in favor of the infants in the betamethasone group were found in the incidence of RDS, 20.7% in the betamethasone group compared with 59.5% in the control group (P less than .005); in the severity of RDS (P less than .05); and in the death rate (P less than .05). A higher incidence of hypoglycemia was found among infants in the betamethasone group (P less than .05). Prolonged rupture of the membranes played no protective role against RDS, and the incidence of infection was similar in both groups.