The oxygen consumption (VO2) of healthy volunteers and patients recovering from respiratory failure in the ICU was measured by indirect calorimetry during complete mechanical (VO2vent) and spontaneous (VO2wean) ventilation. The work of breathing was calculated as the difference in VO2 between spontaneous and mechanical ventilation and expressed as a percentage of VO2vent (% delta VO2). The average % delta VO2 for eight normal healthy volunteers was 3.7 +/- 1.8%, while it was 7.7 +/- 8.8% with the Bechman and Utah metabolic carts for patients recovering from ventilatory failure who were weaned successfully from respiratory support based on clinical criteria within 24 h of their metabolic study. In patients who were not weaned successfully, the average % delta VO2 measured was 24.7 +/- 12.3%. Indirect calorimetry is a relatively simple, reliable bedside technique for determining the oxygen cost of breathing. In our sample, the oxygen cost of breathing was a reliable predictor of weaning and extubation in patients recovering from respiratory failure. This measurement may be clinically useful in identifying the patient who cannot sustain spontaneous ventilation because of excessive respiratory work.