Nitrogen balance and energy expenditure were measured in 18 traumatized and/or septic patients and five depleted patients during different dietary conditions. Total parenteral nutrition (TPN) was given with nonprotein energy entirely as hypertonic glucose solutions (glucose system) or as half glucose-half intravenous fat emulsion (lipid system). In acutely ill patients, the change from 5% dextrose to TPN resulted in a prompt improvement of nitrogen balance to maintenance levels. There were no significant differences between patients given the glucose or lipid system. The five depleted patients were given the lipid and glucose systems alternately for a total of 19 one-week periods. A highly positive N balance, 80 mg N/kg . day, was attained on both diets. There was no significant difference between diets and no period of adaptation after switching from one diet to the other. On comparable intravenous diets, the acutely ill patients had higher plasma concentrations of glucose, glycerol, triglycerides, insulin, and glucagon than did the depleted patients. The study shows that the nitrogen-sparing effects of the lipid and the glucose systems are similar in moderately traumatized or infected as well as in malnourished patients. Factors other than nitrogen balance are of greater importance when choosing between the lipid and the glucose system for intravenous support.