A controlled trial of elective intervention with continuous positive airway pressure (CPAP) was performed on 24 infants with hyaline membrane disease whose arterial oxygen tension (Pao2) fell below 8kPa (60 mmHg) while they were breathing a fractional inspired oxygen concentration (F1O2) greater than 0.60. A face mask was used to apply the CPAP. The progress of the 12 infants who were treated on entry to the trial was compared with that of 12 infants who were treated later. All 12 infants in the early-intervention group and 8 infants in the late-intervention group survived. When CPAP was started, Pao2 increased and the early-treated infants breathed high concentrations of oxygen for a shorter period than the late-treated infants. The 4 infants in the early-intervention group who required mechanical ventilation needed lower mean airway pressures to achieve satisfactory gas exchange than the 7 ventilated infants in the late-intervention group. We conclude that a Pao2 less than 8 kPa while breathing an F1o2 greater than 0.60 is an adequate indication for giving CPAP in hyaline membrane disease, and that early intervention with CPAP allows infants who go on to require mechanical ventilation to be ventilated at lower pressures.