The study developed a new SII-PNI score to predict mortality in elderly patients with community-acquired pneumonia (CAP) admitted to intensive care units (ICUs). From the MIMIC-IV database, 634 patients were included, divided into a training cohort (444 patients) and an internal validation cohort (190 patients), plus an external validation cohort of 149 patients from the Affiliated People's Hospital of Ningbo University (January 2024–March 2025). The optimal thresholds were SII 2030.28 (AUC 0.573, 95% CI 0.517–0.628, p < 0.001) and PNI 0.635 (AUC 0.63, 95% CI 0.584–0.692, p < 0.001) by ROC analysis in the training cohort. The SII-PNI score demonstrated predictive value for 30-day in-hospital mortality with an OR of 2.19 (95% CI 1.62–2.95, p < 0.001). Kaplan-Meier analysis confirmed that patients with a score of 2 had significantly higher 30-day mortality compared with scores of 0 or 1 (p < 0.05) in all cohorts. The model has been validated both internally and externally in geriatric patients with CAP.[3]