Both oxygen consumption index (VO2-index) and simplified acute physiology score (SAPS) are reported to be reliable predictors of the ultimate outcome in critically ill patients. The purpose of this study was to verify whether survivors and nonsurvivors have different VO2-indices and whether the prognostic potency of SAPS can be improved by addition of VO2-index as a supplemental physiological variable. In 50 mechanically ventilated surgical ICU patients with heterogeneous underlying diseases, SAPS was calculated and VO2-index was determined by continuous 24-h measurement of oxygen consumption. The VO2-indices of survivors and nonsurvivors were not significantly different (p greater than 0.05), which is in contrast to the results of earlier studies. This contrast may be explained by a difference both in methods of VO2-measurement and in study populations. SAPS was significantly lower in survivors than in nonsurvivors (p less than 0.005) and was able to classify the patients correctly into groups of increasing probability of death. However, SAPS failed to be a helpful prognosticator in the individual patient. The addition of VO2-index to SAPS as a supplemental physiological variable did not substantially improve the prognostic potency. Because a higher VO2-index did not necessarily indicate a better survival chance, there is no argument for therapeutic interventions aimed exclusively at increasing VO2-index, as suggested previously.