A randomized trial of surfactant replacement therapy at birth was conducted at the University of Rochester between June 1983 and November 1985. Thirty-four premature infants, 25 to 29 weeks' gestational age, received a preventilatory dose of a calf lung surfactant extract in saline prepared at the University of Rochester. A control group of 31 infants received a preventilatory dose of saline alone. The major finding of this trial is that a single preventilatory dose of calf lung surfactant extract reduces the severity of the respiratory distress syndrome during the first 24 hours of life. The beneficial effects, however, are not sustained in many infants and diminish after 24 hours of life. The survival rate was 71% in both the control and surfactant-treated groups. There was a lower incidence of pneumothorax in the surfactant-treated group. There were no differences in the incidence of bronchopulmonary dysplasia, patent ductus arteriosus, and intraventricular hemorrhage. No adverse effects of surfactant replacement therapy were identified. Results of this study suggest that multiple postventilatory doses of surfactant will be required for optimal therapy.