Transportation of critically ill patients requiring ventilatory support represents a common, yet difficult, problem faced by clinicians. We examined 28 patients requiring transport in a prospective, randomized fashion, comparing manual ventilation with ventilation provided by a transport ventilator. Patients were ventilated to their destination with one method and returned with the alternate method. After manual ventilation, all patients showed a marked respiratory alkalosis (pH increased from 7.39 to 7.51 and PaCO2 decreased from 39 to 30 torr). After ventilation with the transport ventilator, no appreciable changes in pH or PaCO2 were seen. Oxygenation remained stable with both methods. No patient suffered hemodynamic instability, although two patients in the manual ventilation group developed cardiac arrhythmias. We conclude that when ventilatory support is required during transport, a transport ventilator produces reliable control of ventilation.