The effect of malnutrition upon body composition was evaluated by determining body composition using multiple isotope dilution in 25 normal volunteers, in 75 patients with clinical malnutrition and in 20 patients who underwent an elective operative procedure of moderate severity. Malnutrition resulted in a loss of body cell mass, accompanied by an expansion of the extracellular mass. In the patients with clinically obvious malnutrition, the size of the body cell mass was reduced to 60 per cent normal, while the extracellular mass was 24 per cent more than normal. By the fifth day following an uncomplicated extensive operation, the body cell mass was reduced by 13.9 per cent, while the extracellular mass was increased by 9.6 per cent. The expansion of the extracellular mass obscures the loss of body cell mass, and therefore, weight loss does not accurately reflect the loss of cellular mass.