Nutritional support is central to prompt treatment of patients with generalized secondary peritonitis (GSP).These patients desperately need a simple and affordable solution to evaluate their daily energy need.Objective: to determine accuracy of estimating the Resting Energy Expenditure (REE) in GSP patients.Materials and methods. Study design: a prospective, single center study. The inclusion criteria: diagnosed GSP and stay at the Intensive Care Unit (ICU). Three treatment arms were formed. The first arm included all patients (n=61), the second arm included patients capable to breath spontaneously and adequately (n=29), and the third arm included patients on artificial ventilation (n=32). Reference values of REE were calculated by Indirect Calorimetry (IC) method using Engstrom Carestation Lung Ventilator and Metabolic (General Electric,USA). Six equations were used to predict REE values: Ideal Body Weight multiplied by 25 (IBWX25); Actual Body Weight multiplied by 25 (ABWX25); J. A. Harris, F. Benedict (HB); HB with corrective ratio 1.25 (HBX1.25); C. Ireton-Jones, 1992 (IJ); PennState, 2003 equation, in modification (PS). SPSS Software Package was used for statistical analysis of the results. The zero hypothesis was rejected at P<0.05.Results. In patients with GSP, the REE value determined by means of Indirect Calorimetry method was equal to 25.78±1.37 kcal/kg/day. If compared with Indirect Calorimetry results, predictive accuracy of calculation equations in the second and third arm, respectively, were as follows: IBWX25: 30 and 0%, HB: 36.7 and 9.9%, HBX1.25: 49.9 and 45.5%, IJ: 51.8 and 53.2%, ABWX25: 63.4 and 60.6%, PS (as determined in patients on mechanical ventilation only): 42.4%.Conclusion. Indirect Calorimetry method is the only accurate way of REE evaluation in GSP patients. ABWX25 and IJ showed the highest predictive accuracy. IBWX25 and HB had the lowest predictive accuracy.