Some patients with chronic cor pulmonale have hypoxemia only during normal daily activity. This can be corrected by portable oxygen. Whether or not the weight of the apparatus (4.2 kg) adds an additional metabolic demand of sufficient magnitude to cancel or outweigh the advantages of portable oxygen is the subject of this report in 6 patients with chronic obstructive pulmonary disease and 2 with kyphoscoliosis. All received standardized treadmill exercise while breathing room air, and the same exercise with a portable liquid oxygen system sitting on the floor or carried by the patients. The additional load created only a 6.7% increase in CO2 production and did not reverse the supplemental oxygen benefit to arterial oxygen saturation under the experimental conditions of the study.