Peripheral blood lymphocytes from normal and thermally injured patients were studied for their ability to form rosettes with sheep red blood cells (SRBC) by active E-rosette assay at different times postinjury. The population of active rosette forming cells (ARFC) was observed to be significantly lower (P less than 0.001) in thermal injury patients compared with the normal subjects. The age and sex of the patient did not seem to have any significant effect on the ARFC population or the total lymphocyte count. The degree of burn had a significant (P less than 0.05) effect on the ARFC population but not the total lymphocyte count during the first 10 days immediately after burn, with partial skin thickness burn patients having more ARFC compared with full skin thickness burn patients. Further analysis of the data revealed an important correlation between the per cent ARFC in peripheral blood and the survival of the patients. Patients who died from their injuries showed a continuous decline in ARFC over the period of study, whereas the patients who survived showed a gradual increase until the ARFC number returned to within the normal range. The observed differences in the ARFC population between survivors and non-survivors were apparent as early as the first 10 days postinjury. Thus, the percentage of ARFC in the total peripheral blood may be used as an index for the early identification of patients at a greater risk of mortality.