Septic shock is characterized by lymphocyte alterations associated with increased risk of nosocomial infections and mortality. IL-7, a cytokine required for T cell survival, is thought as a novel therapy for septic patients with severe lymphopenia. We assessed CD4⁺ lymphocyte responsiveness to rhIL-7 in septic shock patients ex vivo. Thirteen septic shock patients and 10 controls were included. The MFI of pSTAT5, a key signaling molecule for IL-7, was measured by flow cytometry in CD4⁺FOXP3− (T_effs) and CD4⁺FOXP3⁺ (T_regs) lymphocytes after whole-blood incubation with increasing doses of rhIL-7. The basal level of pSTAT5 in nonstimulated T cells was higher in patients. However, the maximal activation level in response to the highest doses of rhIL-7 was similar in both groups. Importantly, low doses of rhIL-7 preferentially activated T_eff versus T_reg in patients and nonsurvivors tended to present with decreased pSTAT5 expression. This pilot study is the first to highlight, in septic patients, the interest of pSTAT5 measurement in whole blood for the monitoring of rhIL-7 therapy. Such a method could represent an innovative, biologic tool for monitoring leukocyte pharmacological responses to biotherapies in daily clinical practice in other clinical contexts.