Immunological assessment is important to characterize the host defence response of trauma patients as infection is the most common cause of severe morbidity and late death. Sixty trauma patients were followed serially and divided into three groups: those with an uneventful recovery (n = 17), those with recovery after major sepsis (n = 27) and those who died (n = 16). The ability of peripheral blood monocytes to express the antigen HLA-DR was measured and compared to the results from 77 asymptomatic volunteers. After initial injury, there was a significant reduction from normal in the three trauma groups. It took one week for HLA-DR antigen expression to return to the normal range in the first group, three weeks in the second group, and in the third group it never returned to normal. Monocyte HLA-DR antigen expression, after incubation with lipopolysaccharide, distinguished those patients who survived from those who died. There was no difference in HLA-DR antigen expression between a high transfusion group of 31 patients who received 10 or more units of blood and a low transfusion group of 29 patients who received less than 10 units. The ability of monocytes to express HLA-DR antigen correlated directly with the clinical course in these patients and its measurement identified a group of patients at high risk of infection and death following trauma.