We have made a study of the energy requirements of liver transplant patients in the immediate post-operative phase, by comparing different methods. A study of energy use was made with indirect Calorimetry (IC), of calculation of Resting Energy Expenditure (REE) according to Fick's formula modified by Liggett, and of the calculation of basal energy Expenditure (BEE) applying Harris-Benedict's equation (HB). The correlation between the REE calculation using indirect calorimetry and that of BEE using Harris-Benedict (r = 0.7567) did not give a correction factor, applied to the Harris-Benedict formula) (REE by IC/BEE), which was uniform for all patients, oscillating as it did between 1.0 and 1.8. We found no correlation between REE by IC and that calculated using the modified Fick method, nor between the modified Fick method and BEE as calculated by Harris-Benedict. Our conclusion is that, with the IC method as reference to evaluate energy use, the Harris-Benedict calculation appears to be more reliable than that using the modified Fick equation and that, in the group of patients studied, and given the individual variations in the REE ratio calculated by IC and the BEE calculated by HB, we were unable to find a suitable correction factor for them all. IC is therefore the ideal method for evaluating energy use in critical patients in the ICU.