Systemic Immune-Inflammation Index (SII) is a hemogram-derived biomarker reflecting the balance between systemic inflammation and immune response. In critically ill patients, inflammatory burden at admission may play a key role in prognosis. This retrospective cohort study aims to evaluate the association between admission SII levels and intensive care unit (ICU) mortality in adult patients admitted to a reanimation ICU. Secondary objectives include comparison of mortality according to infectious versus non-infectious admission diagnoses and assessment of the relationship between SII and established severity scores, including APACHE II and Glasgow Coma Scale (GCS). The findings may support the use of SII as a rapid, inexpensive prognostic marker in critically ill patients.