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To assess the levels of expression of the antiapoptotic gene Bcl-2 and the proapoptotic gene Bax in circulating mononuclear cells (CMNC) harvested during the course of severe sepsis (SS) in formerly non-immunocompromised patients undergoing hospital-acquired infection, in parallel to cytokine levels. Prospective study. Intensive care unit. A total of 24 patients without immunodeficiency undergoing standard goal-directed therapy for nosocomial SS, 10 critically ill patients without sepsis, and 10 healthy controls. Blood was collected before infection and within 12 h, 1, 3 and 7 days after fever onset, to determine plasma concentrations of IL-6, IL-10, TNF-alpha, C-reactive protein, whole blood cell counts, lymphocyte subsets, annexin V labelling for apoptosis, and Bax and Bcl-2 relative RNA expression by real-time polymerase chain reaction. SS patients displayed increased cytokine concentrations, TNF-alpha being significantly increased at full-blown sepsis. Within 12 h after onset of infection, lymphocyte counts were lower in SS patients than in critically ill controls ( p=0.001), and this phenomenon was marked in CD4+ and CD8+ subsets ( p<0.001). This was associated with enhanced apoptosis in CMNC (15.7+/-8.7% vs 3.4+/-2.1%, p<0.001) and a significant down-expression of the Bcl-2 gene throughout the study ( p<0.05). In contrast, the expression of Bax did not change significantly. Within 12 h of fever onset, non-survivors expressed a 10-fold down-expression of Bcl-2 when compared to survivors ( p<0.001). An early transient down-expression of the gene Bcl-2 occurred in CMNC harvested from SS patients who died despite intensive care. In contrast, the expression of the gene Bax did not change significantly.