From September 1989 to September 1990, a total of 25 newborn infants with severe RDS (on mechanical ventilation and with a FiO2 above 0.6) were treated with porcine surfactant (Curosurf), since its efficacy has been demonstrated in a prior clinical trial (Pediatrics 1988; 82: 683-91). The mean birth weight and gestational age were 1,327 +/- 566 g and 29.5 +/- 3.8 weeks, respectively. Twenty-four percent of the babies had an arterial pH below 7.1. At a mean postnatal age of 9.1 +/- 10.7 hours, a dose of 200 mg/Kg of Curosurf was given by the tracheal route. Mean paO2 rose from 47 +/- 14 mm Hg to 166 +/- 64 mm Hg (p less than 0.001). The paO2 increased in 24 of the 25 cases and in 20 (80%) the post-treatment paO2 was a least two times higher than the pre-treatment value. An hour after surfactant treatment, the paCO2 decreased from 58 +/- 21 mm Hg to 45 +/- 16 mm Hg (p less than 0.01) and the pH increased from 7.19 +/- 0.15 to 7.28 less than 0.11 (p less than 0.001). The following complications were observed: pulmonary interstitial emphysema (16%), pneumothorax (8%), patent ductus arteriosus (36%), intraventricular hemorrhage (28%) and bronchopulmonary dysplasia (24%). The neonatal survival rate and the survival rate after discharge from the hospital were 76% and 64%, respectively. The combined survival and absence of DBP was 32%. Nonsurvivors had lower birth weights and gestation ages, as well as lower paO2 5 min after treatment and higher FiO2 24 hours after surfactant treatment than did the survivors.