In critically ill patients, oxygen consumption (VO2) and delivery (DO2) are used to determine optimal haemodynamic management and to grade severity of illness. VO2 may be measured by indirect calorimetry with metabolic gas monitoring systems or derived using the reverse Fick principle. Oxygen saturation (SaO2) may be measured directly by co-oximetry or derived by equations for incorporation into reverse Fick equations. A prospective study comparing VO2 measured by these methods was performed in 20 critically ill patients. The mean VO2 measured by the metabolic gas monitoring system (308 +/- 63.9 ml/min) was significantly greater than that measured by reverse Fick using measured SaO2 (284 +/- 72.0 ml/min) (P < 0.01). This difference may be due to intrapulmonary VO2. When SaO2 was calculated from three logarithmic equations and incorporated into the reverse Fick equations, calculated VO2's were significantly greater (P < 0.001) than those measured by indirect calorimetry. Correlation was poor and wide limits of agreement (-118 to +350 ml/min) were demonstrated. VO2 should ideally be measured by indirect calorimetry in the critically ill, or if reverse Fick is used, SaO2 should be measured by co-oximetry as the use of equations for clinical measurement of SaO2 is clinically suspect.