We measured the oxygen cost of breathing (VO2resp) in 13 patients with cardiorespiratory disease requiring artificial ventilation as the difference between the oxygen consumption during spontaneous respiration (VO2tot) and that during artificial ventilation (VO2nonresp). Average VO2tot was 312 +/- 90 ml/min (mean +/- 1 SD), whereas VO2nonresp was 246 +/- 38 ml/min (p less than 0.01). Average VO2resp was 75 +/- 82 ml/min (range, 8 to 286), representing 24% of VO2tot or 8.7 +/- 8.9 ml O2/L ventilation. In normal resting adults, VO2resp in 5 to 10 ml/min, about 1 to 3% VO2tot, or 0.25 to 2.5 ml O2/L ventilation. The VO2resp had an exponential relationship with FEV1 (r = 0.97) in 6 patients who had had prior spirometry. Because minute ventilation was normal in our patients (8.8 +/- 2.2 L/min), the elevated VO2resp represented an increase in the work of breathing as well as a decrease in the efficiency of the respiratory muscles. These results suggest that, where O2 transport is compromised, artificial ventilation may release substantial quantities of oxygen for use by other body systems.