Objective: To determine if Apache II Trauma Score or obesity correlate with measured resting energy expenditure (REE) and influence the validity of calculated REE. Design: Descriptive case series. Setting: Hospitalized care at an urban medical center. Patients: Consecutive sample of sixty-six patients having indirect calorimetry measurements. Analysis: Patients were assigned Apache II Scores and classified by body weight (15 obese, 25 normal weight and 11 underweight patients). Estimates of REE were calculated using Harris-Benedict and Mifflin equations. Main Outcomes: Measured REE did not correlate significantly with Apache II scores. Correlation of Harris-Benedict calculated REE with measured REE was moderate (r = 0.54, p < 0.001) and did not improve with controlling for obesity (use of ideal/adjusted body weight, use of Mifflin equation or with obese patients excluded). Conclusion: Calculations for REE in hospital patients are unlikely to be improved by incorporation of Apache II scores. Obesity does not affect the validity of calculated REE.