This report documents a specific situation in which indirect calorimetry produced large overestimations of oxygen consumption and energy expenditure of a patient on a respirator. High positive-end expiratory pressures combined with high fractional concentrations of O2 led to perforation of lung parenchyma with considerable loss of O2, which produced subcutaneous emphysema and recurrent pneumothoraces. Typical calculations for determining energy expenditure from O2 consumption, therefore, could not be employed. Calculations utilizing the volume of CO2 expired, however, were less sensitive and variable during this time period. Resting energy expenditures were calculated from the volume of CO2 expired by using 5.52 kcal/l as the caloric equivalent of CO2. This procedure does not overestimate energy needs when a physiological gas leak exists (high positive-end expiratory pressures and high fractional concentrations of O2 or chest tube). This is very important in the ventilatory patient as excess carbohydrate can further stress the compromised pulmonary status.