19 preterm infants with severe respiratory distress syndrome (RDS) were treated with a single dose of natural porcine surfactant (Curosurf, 200 mg/kg). 9 patients had a birth weight of less than 1000 g (845 +/- 112 g, mean +/- SD and the mean gestational age was 27.2 +/- 2.1 weeks). The other 10 had a birth weight of greater than 1000 g (1521 +/- 218 g and a mean gestational age 31 +/- 2.8 weeks). Age at treatment was 3 h in infants less than 1000 g and 4 h in patients greater than 1000 g. Both groups of infants showed a rapid improvement in oxygenation and gas exchange within minutes after surfactant replacement. Exposition to greater than 60% and greater than 40% oxygen was identical in both groups. However, time in greater than 21% oxygen was significantly longer in infants less than 1000 g (median 30 days, 8.5 days in patients greater than 1000 g, p less than 0.01). The duration of mechanical ventilation was 33 days, in patients greater than 1000 g and 5 days; p less than 0.01. None of the infants developed a pneumothorax, but 6 out of 9 patients less than 1000 g developed mild bronchopulmonary dysplasia. 2 infants less than 1000 g died at day 5 and day 11 from cardio-circulatory arrest following ligation of a patent ductus arteriosus, and nosocomial septicaemia, respectively. Prolonged mechanical ventilation and exposure to oxygen in patients less than 1000 g cannot be attributed to surfactant deficiency alone.(ABSTRACT TRUNCATED AT 250 WORDS)