It is current clinical practice to give intravenous nutrition (IVN) to critically ill postoperative septic intensive care patients to prevent loss of body protein, although it has not hitherto been possible to confirm this by direct measurement of body composition. Using a neutron activation analysis facility adapted to provide an intensive care environment and tritiated water dilution we directly measured total body water, protein and fat before and after 10 days of IVN (mean daily non-protein energy and amino acid intakes 2,750 kcal and 127 gm) in eight adult intensive care patients. All patients had recovered from the septic shock syndrome but were still ventilator dependent at the start of IVN. Six patients survived to leave hospital. As a group, the patients lost 12.5% of body protein (mean loss 1.5 +/- SE 0.3 kg; p = 0.001) despite a gain in fat (mean 2.2 +/- 0.8 kg; p = 0.026). There were, in addition, large losses of body water in most patients (mean, 6.8 +/- 2.6 kg; p = 0.036). We conclude that substantial losses of body protein occur in critically ill septic patients despite aggressive nutritional support and that further research is urgently required on the fate of infused substrates and the efficacy of alternative nutritional therapies.