In acutely ill patients both hypo- and hyperalimentation must be avoided by adjusting caloric intake to total energy expenditure (TEE). We determined the discrepancy between basal energy expenditure (BEE) calculated from the basic Harris-Benedict formula and TEE measured by continuous indirect calorimetry in a heterogeneous group of mechanically ventilated surgical patients. We also compared the accuracy of TEE calculated from the corrected Harris-Benedict formula or estimated by intermittent indirect calorimetry to that of TEE measured by continuous indirect calorimetry. The poor correlation between calculated BEE and measured TEE was significantly (p less than .05) improved by a correction factor based on each patient's clinical condition. The mean absolute difference between calculated TEE and measured TEE was 8.9 +/- 9.6 (SD) %. Calculations were significantly (p less than .05) improved by estimating TEE from two 5-min recording periods, which suggests that continuous indirect calorimetry may not always be necessary to guide caloric replacement.