To determine the frequency of hemodynamic and respiratory complications during movement within the hospital, we conducted a prospective study involving 36 critically ill, ventilator-dependent patients who needed procedures done outside the intensive care unit. During the first 20 transports, patients received ventilation through a manual resuscitation bag. Arterial blood gas measurements showed frequent changes from baseline with alterations in PCO2 (greater than 10 torr) or pH (greater than 0.05) occurring on 14 occasions. In a subsequent study, 16 patients received ventilation during transit with the aid of a portable mechanical ventilator. Although 6 patients showed changes in arterial blood gas values, mean changes in PCO2 and pH were significantly less than in the group that received manual ventilatory support. (p less than 0.01). Hemodynamic complications of hypotension and cardiac arrhythmia showed a significant correlation with disturbances in arterial blood gases (p less than 0.05). Although limited by the lack of a control period, this study shows that the transport of critically ill patients may result in severe hemodynamic complications; it also suggests that these complications might be prevented by more careful monitoring of ventilation.