These studies were undertaken to investigate the effect of acute bacterial infections on the absolute number of peripheral blood lymphocytes (PBL) in an elderly population and to evaluate the prognostic significance of a decreased number of PBL in critically ill aged patients. The results show that a significant lymphopenia develops in elderly patients during the course of an acute bacterial infection whereas the same type of acute illness has no effect on the PBL count of younger subjects. The lymphopenia is not related to a particular localization of the infection nor to the type of bacterial pathogen. The prognosis of the bacterial infection is closely linked to the severity of the lymphocyte depletion and its outcome can nearly be predicted by monitoring the variation of the number of circulating lymphocytes during the early course of the disease.