Objective To investigate the energy expenditure of mechanically ventilated patients,compare the measured energy expenditure (MREE) with the energy expenditure expected from the Harris-Benedict equation adjusted with correction factors (PREE). Methods Twenty-four critically ill adult patients who were mechanically ventilated in the intensive care unit were enrolled in this study. Data during the 72 hours of mechanical ventilation were collected for computation of severity of illness. Resting energy expenditures were derived at 72hours after mechanical ventilation by indirect calorimetry. Predicted basal energy expenditure was obtained at the same time using the Harris-Benedict equation and predicted resting energy expenditure was calculated using the Harris-Benedict value adjusted with correction factors for illness. Results The Acute Physiology and Chronic Health Evaluation Ⅱ (APACHE Ⅱ)scores and Marshall scores were 14 ± 5 and 6 ± 3, respectively. MREE and PREE were (6 793.64 ± 1 197.15) and (8 041.02 ± 1 971.54) kJ/day, respectively. There was no correlation between MREE and PREE (r2 = 0. 28, P = 0. 07), and the difference between MREE and PREE was statistically significant (t = 7.62, P = 0.04). No statistically significant correlations were observed between both MREE or PREE and APACHE Ⅱ score or Marshall score (r2 = 0. 14, P = 0. 08; r2= 0. 08, P = 0. 63; r2 = 0. 05, P =0. 65; r2 = 0.03, P = 0. 87, respectively). Conclusions In mechanically ventilated patients, the energy expenditure is not correlated with the severity of illness. The Harris-Benedict prediction modified with correction factors for severity of illness systematically overestimates the total energy expenditure.