Infection remains a major cause of morbidity and mortality in intensive care medicine. The increased susceptibility of the severely injured patient to sepsis and consecutive multiorgan failure has been attributed to abnormalities in cell-mediated immunity. The purpose of our study was to determine changes in the pattern of lymphocyte subpopulations in severely injured patients and to relate these changes to any development of sepsis and to outcome (indirect immunofluorescence with monoclonal antibodies). During 14 months we investigated 28 patients (ages 15-65 years) suffering from severe multisystem trauma (22 cases) or diffuse peritonitis (6 cases), 6 of whom (21.4%) developed sepsis and multiorgan failure; 4 of these 6 septic patients died. According to the clinical data, patients developed sepsis between the 3rd and 6th days after trauma. We therefore defined days 1-3 as the preseptic phase, days 3-6 as the phase of sepsis development, and days 4-10 as the phase of septic disease. In the preseptic phase there was no statistically significant difference in the pattern of the eight lymphocyte subpopulations measured between patients who later developed sepsis and those who did not. During the phase of sepsis development, however, the patients who did develop sepsis showed significantly reduced numbers of CD2-, CD8-, and CD20-positive cells (P = 0.0003; P = 0.009; P = 0.012). The number of helper cells (CD4) was also decreased, but the difference between the two groups failed to reach statistical significance (P = 0.08).(ABSTRACT TRUNCATED AT 250 WORDS)