We describe a simple and reliable method to determine optimal airway pressure in infants with idiopathic respiratory distress syndrome who require continuous positive airway pressure treatment. Esophageal pressure was monitored in ten infants with IRDS during initial application of CPAP. As the level of CPAP was increased in 2 cm H2O increments, changes in Pes were compared with changes in PaO2. Below optimal airway pressure, Pes as well as PaO2 increased insignificantly. When optimal airway pressure (8.1 +/- 0.8 cm H2O) was applied, there was a marked increase in Pes (3.6 +/- 0.8 cm H2O. p less than 0.001) and PaO2 (39.0 +/- 10.0 mm Hg, p less than 0.01). Further increase in CPAP did not result in any subsequent appreciable increase in Pes while PaO2 decreased slightly and PaCO2 increased. Less than optimal CPAP increases F102 requirements and may increase the associated risk of bronchopulmonary dysplasia, while excessive levels of CPAP may increase the risk of pneumothorax. We suggest that esophageal pressure be monitored routinely to determine the optimal level of CPAP for each infant during the initial application of therapy.