Objectives To determine the time course of plasma immunoglobulin E (IgE) concentration increases after traumatic injury, if increased IgE concentrations were related to clinical events or complications, and if increased peripheral eosinophil counts could be related to trauma, sepsis, or organ-specific complications. Design Data relating to severity of injury, clinical complications, plasma concentrations of IgE, and peripheral eosinophil counts were prospectively collected. Setting Trauma service, tertiary-care medical center. Patients One hundred adult trauma patients admitted to the intensive care unit. Interventions None. Measurements and Main Results Plasma IgE concentrations increased in most patients. However, the greatest increases were observed in patients with sepsis (p = .03), renal dysfunction (p = .04), or pneumonia (p = .02). IgE increases were not related to severity or mechanism of injury, allergy history, or age. The day of highest observed IgE concentration was related to the day of onset of sepsis (p = .012, r = .39), and occurred a mean of 3.8 days after sepsis. Most patients had increased peripheral eosinophil counts and eosinophil percentages of white blood cells during their intensive care unit stays. Eosinophil counts were greater in patients with sepsis (p < .0001), severe sepsis (p < .0001), or pneumonia (p < .002). Conclusions Increased IgE concentrations and eosinophil counts were found after sepsis and do not appear to be related to the initial injury. Since IgE and eosinophil production are enhanced by interleukin-4 and interleukin-5, respectively, these findings suggest that T-helper lymphocyte type 2 cytokines are activated in response to sepsis after traumatic injury. (Crit Care Med 1998; 26:465-469)