Randomized, placebo-controlled. Singleton infants were assigned to receive a placebo (air), prophylactic surfactant treatment given intratracheally, or rescue surfactant treatment. Multiple birth infants received either prophylactic or rescue treatment. Of 282 potentially eligible infants, 246 received treatments at birth and 200 had respiratory distress syndrome and received the full course of surfactant therapy. Preterm infants randomly assigned to receive prophylactic treatment received surfactant soon after birth; those assigned to receive rescue surfactant had instillation at a mean age of 220 minutes if the lecithin-sphingomyelin ratio was _ 2.0 and no phosphatidylglycerol was detected in either amniotic fluid or initial airway aspirate, oxygen requirements were a fraction of inspired oxygen of > 0.5 and mean airway pressure was _ 7 cm H20 from 2 to 12 hours after birth. Up to four treatment doses were permitted within 48 hours; approximately 60 percent of surfactant-treated infants required two or more doses. Endpoints included the mortality rate at 28 days of age, the incidence of bronchopulmonary dysplasia at 28 days after birth and at 38 weeks to adjust for differences in gestational age, the incidence of pulmonary air leaks, and the severity of respiratory distress syndrome as assessed by requirement for supplemental oxygen and mechanical ventilation.