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• Cytotoxic cells decreased but activation increased in septic shock patients. • The function of toxic cells 2–3 days after admission indicated the clinical outcome. • Dynamic monitoring of cytotoxic cells in patients with septic shock was key. : Septic shock is a great threat to human life. Our aim is to explore the immune status and dynamic changes of circulating cytotoxic cells in septic shock patients. : Forty-eight septic shock patients (9 non-survivors and 39 survivors) and 30 healthy controls (HCs) were enroled in our study. The function of cytotoxic cells was dynamically monitored by flow cytometry. : The number of circulating CD8+ T and NK cells decreased significantly in septic shock patients, while the number of CD8+ T cells rose in survivors 5 days after admission. The frequency of HLA-DR+CD8+ T / NK cells increased in both groups after admission but decreased in non-survivors on day 3. Moreover, the frequency of GrA+/GrB+/perforin+NK and GrB+CD8+ T cells decreased to varying degrees in both groups, and the frequency of GrB+/perforin+CD8+ T cells on the second day of non-survivors was significantly lower than that of survival patients. Besides, the frequency of CXCR3+CD8+ T / NK cells was decreased in both groups and remained low in non-survivors, but remarkably increased in survivors after day 3. And the concentrations of cytokines IL-6, IL-10, TNF-α and IFN-γ were significantly increased in septic shock patients. : Circulating CD8+ T and NK cells reduced but activation function was compensatory enhanced in septic shock patients. The frequency of GrB+/PFP+CD8+ T and CXCR3+CD8+ T / NK cells may predict the progression of septic shock patients 2–3 days after admission. [ABSTRACT FROM AUTHOR]