The course of idiopathic respiratory distress syndrome (IRDS) treated with continuous positive airway pressure (CPAP) was studied in 38 infants with a respiratory index (RI) based on AaDO2 and Po2 measurements. Thirteen infants were treated with early CPAP (Fio2= 0.3, Po2 > 50 torr (6.7k Pa)) at a mean age of 7.1 h and 25 infants received late CPAP (Fio2 = 0.5, Po2 > 50 torr) at a mean age of 15.1 h. Significant differences were demonstrated between the two groups in duration of CPAP (42 versus 72 hours) peak RI (3.7 vs. 6.7), time to peak RI from start of therapy (10.0 versus 19.4 h), number of infants ventilated (0 versus 5) and number of air leaks (0 versus 3).The rate of disease worsening as measured by changes in Rl/h before CPAP and after CPAP initiation was comparable in the respective treatment groups.