Critically ill patients admitted to the intensive care unit with severe sepsis or major blunt injury undergo massive changes in body composition. We compared these changes in 12 patients with generalized peritonitis, and in 18 patients with major blunt injury over a 21-day period soon after their admission. Body composition was measured as soon as the patients were hemodynamically stable, and again 5, 10, and 21 days later. In both groups, losses in total body protein (TBP) were greatest over the first 10 days. TBP lost over the study period averaged 13.1 +/- 1.3 (SEM)% for the sepsis group, and 14.6 +/- 1.3% for the trauma group. Total body water (TBW) lost postresuscitation averaged 11.1 +/- 1.3 L and 6.7 +/- 1.1 L for the two groups, respectively, these changes largely being accounted for by changes in extracellular water (ECW). Our results demonstrate a striking similarity in the changes in total body protein for these two groups of critically ill patients. The sepsis patients retained approximately twice the volume of fluid of those with major trauma.