It is widely believed that increased hydration of the fat-free body accompanies most major disease processes as a result of contraction of the body cell mass and expansion of the extracellular fluid. Measurements of total body water (TBW) and total body nitrogen in 68 normal volunteers and 95 surgical ward patients presenting for intravenous nutrition have been used to derive ratios of TBW to fat-free mass (TBW:FFM) and protein indices (PI), where PI is defined as the ratio of measured total body protein to predicted TBP. Mean values of PI were 1.009 +/- 0.116 (SD) and 0.783 +/- 0.152 in the normal and patient groups, respectively, corresponding to mean TBW:FFM ratios of 0.719 +/- 0.016 and 0.741 +/- 0.029. However, 48 patients had normal TBW:FFM despite having lost 15% of body protein. A theoretical model of body composition changes in catabolic illness is presented, which is in accord with the patient data, demonstrating that TBW:FFM does not necessarily increase in catabolic illness and that the ratio masks underlying shifts in body fluid compartments.