Accurate assessment of energy requirements of humans under various physiological, pathological, and environmental conditions is hampered by the high cost of techniques for measuring daily energy expenditure. A room respirometer novel in its novel in its low cost of construction and simplicity of operation is described, together with an appreciation of errors and assumptions. It is suitable for measurements of daily energy expenditure in humans with an accuracy of +/- 1.5% and can be reproduced without difficulty in both developed and developing countries.