A retrospective analysis of 55 mechanically ventilated critically ill patients was conducted to determine adequacy of nutritional support (total parenteral nutrition or enteral nutrition) according to requirements established by indirect calorimetry. Patients who received 90% to 110% of the established energy requirements as measured by indirect calorimetry were defined as adequately fed. At the time of the indirect calorimetry measurements, all patients were receiving their targeted nutritional support, as assessed by the unit dietitian, who used predictive formulas to assess patients. Indirect calorimetry results showed that 25% of the patients were overfed (receiving >110% of energy requirements), 35% were underfed (receiving <90% of energy requirements), and 40% were adequately fed (receiving 90% to 110% of energy requirements). We determined that critically ill patients with a body mass index <20 kg/m2 were the most likely group to be assessed inappropriately by available regression equations. If indirect calorimetry measurement is unavailable, we suggest using an empiric formula of 37 kcal/kg for critically ill patients with a body mass index <20 kg/m2.