BACKGROUND: The use of steady state as the endpoint for performance of indirect calorimetry (IC) is controversial. We designed this prospective study to evaluate the necessity and significance of achieving steady state. METHODS: Patients with respiratory failure placed on mechanical ventilation in a short- or long-term acute care unit at any 1 of 3 university-based urban hospitals were eligible for the study. The 24-hour total energy expenditure (TEE) was determined by a Nellcor Puritan Bennett 7250 continuous IC monitor. Measured gas exchange parameters were obtained and averaged every 1 minute for the initial hour and then every 15 minutes for the next 23 hours. Over the initial hour, resting energy expenditure (REE) was averaged for intervals over the first 20, 30, 40, and 60 minutes, and for various definitions of steady state where oxygen consumption (VO2) and carbon dioxide production (VCO2) changed by or = 30-minute duration. In those patients with greater variation (CV for VO2 >9.0), interval testing of at least 60 minutes or more was required to attain levels of correlation similar to that achieved by the steady-state period defined by the most stringent criteria. CONCLUSIONS: These data support the use of steady state, best defined as an interval of 5 consecutive minutes whereby VO2 and VCO2 change by 9.0, test duration of at least 60 minutes may be required. These latter patients should be considered for 24-hour IC testing.