Introduction: To investigate the value of contrast-enhanced CT findings - splenic and aortic radiodensities and their ratios (spleno-aortic ratio) - in predicting the prognosis of critical care unit patients (CCU). Methods: One hundred thirteen continuous CCU patients with an acute deterioration (Group A: 37 women, age: 67.2 ± 14.0 years) were included in the retrospective study. Radiodensities of the spleen and aorta were evaluated by two radiologists separately. The spleno-aortic ratio was calculated. Matthews correlation coefficient (MCC) was used in conjunction with receiver operating characteristic analysis (ROC) to assess if and which parameter was most suitable for short-term mortality prediction. The intra-class correlation coefficient assessed consensus across readers. To validate the results for the best predictor, a second cohort was evaluated (Group B: 354 CT scans). Results: The portal venous spleno-aortic ratio was best suited to predict 72-hour mortality (AUC = 0.91). A threshold ratio ≤0.53 predicted short-term mortality with a high sensitivity (80.95%) and specificity (96.74%, MCC = 0.79). The post-test probability was 85%, assuming a pre-test probability of 18.6% (72-hour mortality rate). ICCs of HU measurements in the aorta, spleen, and its ratios showed high interrater agreement (ICC: 0.92–0.99). In a control cohort, a threshold ratio ≤0.53 predicted CCU patientś outcome satisfactorily (SENS = 83.93%, SPEC = 97.65%, PPV = 87.00%, NPV = 97.00%). Conclusions: The portal venous spleno-aortic ratio serves as a distinctive imaging feature to predict short-term mortality. For CCU patients with a cut-off portal venous spleno-aortic ratio ≤0.53, the risk of dying within three days after CT scan is approximately twenty times higher.