A 5-year-old child required a prolonged period of ventilatory assistance, provided in the form of pressure support ventilation. There was a significant negative correlation between the level of pressure support and the PaCO2. Requirements for sedation were reduced with pressure support compared with conventional controlled ventilation. Changing the ventilator trigger level from -1 cm H2O to -3 cm H2O did not affect PaCO2. Satisfactory arterial blood-gas tensions were obtained only when the inspiratory pressure was increased to 10 cm H2O above positive end-expiratory pressure.