Objective To assess and compare the resting energy expenditure measured by indirect calorimetry (MREE) and calculated with Harris-Benedict formula adjusted with correction factors (CREE) in critically ill surgical patients receiving mechanical ventilation,and to evaluate the relationship between resting energy expenditure and the severity of diseases.Methods From August 2008 to February 2010,21 patients fitting the inclusion criteria were selected into the present study.The data of the patients were collected to calculate acute physiology and chronic health evaluation Ⅱ score ( APACHE Ⅱ score) and multiple organ dysfunction score ( Marshall score).MREEs were measured using indirect calorimetry of a MedGraphics CCM/D System,and CREEs were calculated at the same time with the Harris-Benedict formula.Results Within the week of nutrition support,the mean CREE of the 21 patients was significantly higher than the mean MREE [ ( 8305.09 ± 1392.76 ) kJ vs.(6544.84 ±2079.65) kJ,P =0.000].The differences between MREE and CREE were statistically significant on the 0 ( P =0.000),1 ( P =0.000 ),2 ( P =0.000 ),and 4 day ( P =0.003 ) of nutritional support.There was no correlation between MREE and CREE (r =0.064,P =0.408 ),nor between MREE and APACHE Ⅱ ( r=-0.045,P =0.563 ).There was a correlation between MREE and Marshall score (P =0.001 ),but the correlation coefficient was low ( r =0.263).Conclusions The Harris-Benedict prediction modified with correction factors for severity of diseases overestimates the resting energy expenditure of critically ill surgical patients.Indirect calorimetry is a more accurate method for determining resting enenrgy expenditure.