ABSTRACT. End-stage renal failure (ESRF) and chronic hemodialysis (HD) induce a state of immunodeficiency that involves T cell-mediated responses. A decreased T cell number combined with a reduced T cell lifespan and an increased T cell activation might play a role in the immune impairment associated with ESRF and chronic HD. Increased T cell activation associated with immunodeficiency suggests that activated T cells may be driven to apoptosis. To test this hypothesis, CD3+ T cell activation (CD69) and apoptosis (annexin V, CD95 (Fas), and DNA fragmentation) were analyzed in a case control study after blood draw sampling (ex vivo), in culture conditions, and after phytohemagglutinin or anti-CD3 stimulation. Ex vivo evaluation of T cells showed an increased number of activated CD69+ T cells in chronic HD patients (142 +/- 5 cells/mm3) compared with patients with ESRF (115 +/- 2 cells/mm3, P = 0.04) and controls (74 +/- 2 cells/mm3, P = 0.0006). These data were confirmed in culture conditions and after stimulation. Similarly, annexin V and CD95 (Fas)-positive T cells were more numerous in both patient groups than in controls, irrespective of the experimental conditions (P < or = 0.005 for both markers), and their percentage was always significantly higher in chronic HD patients than in patients with ESRF. The amount of DNA fragmentation was also significantly higher in the cultured resting T cells of chronic HD patients (37 +/- 3%) than in those of patients with ESRF (25 +/- 3%) and controls (20 +/- 2%) (P = 0.01). Percentage of cultured resting T cells expressing both CD69 and annexin V markers was higher in chronic HD patients (17 +/- 4%) than in patients with ESRF (10 +/- 4%) and controls (6 +/- 2%), (P = 0.005). After stimulation (phytohemagglutinin or anti-CD3), CD69+ T cell apoptosis increased by 2.4-fold in chronic HD patients compared with 1.8-fold in patients with ESRF and only 1.2-fold in controls (P = 0.001). T cells from chronic HD patients and patients with ESRF thus showed an aberrant state of early activation that contrasted with an increased proportion of annexin V and CD95 (Fas)-positive T cells engaged in apoptosis, as confirmed by DNA fragmentation. Increased susceptibility to early activated T cell apoptosis is not only associated with uremia, but is also enhanced by HD procedure. This may account for the T lymphopenia, progressive immunodeficiency, and increased infection risk seen in these patients.