OBJECTIVE To compare measurement of oxygen consumption (VO2) by spirometry and the reversed Fick method. METHODS Within-patient comparison using simultaneous measurements by the two methods, one previously calibrated on a metabolic simulator. METHODS Twenty sets of observations on eight patients (57 to 83 yrs) requiring mechanical ventilation in a critical care unit. METHODS None during or immediately before the measurements. RESULTS Duplicate pairs of measurements of VO2 were made with a previously validated spirometric technique and the reversed Fick method (Qt[CaO2 - CVO2]), where Qt is cardiac output, CaO2 is arterial oxygen content, and CVO2 is mixed venous oxygen content. The coefficient of variation of the difference between duplicate measurements by the former technique was only 2.53% compared with 10.4% for the latter. The mean VO2 measurement by the spirometric method was 285.7 +/- 40.7 (SD) mL/min standard temperature and pressure, dry (STPD) and for the reversed Fick method, the mean VO2 measurement was 249.3 +/- 38.5 mL/min STPD. The mean difference was 36.4 +/- 28.5 mL/min STPD (p less than .001). CONCLUSIONS The repeatability of the spirometric method was four times better than the reversed Fick method. The latter gave a significantly lower value that probably, in part, reflects the VO2 of the lung, which is included in the spirometric method but not in the reversed Fick measurement.