AIM: Frail individuals may be at higher risk of death from a given acute illness severity (AIS), but this relationship has not been studied in an English National Health Service (NHS) acute hospital setting. METHODS: This was a retrospective observational study in a large university NHS hospital in England. We analyzed all first non-elective inpatient episodes of people aged ≥75years (all specialties) between October 2014 and October 2015. Pre-admission frailty was assessed with the Clinical Frailty Scale (CFS) of the Canadian Study on Health & Aging, and AIS in the Emergency Department was measured with a Modified Early Warning Score (ED-MEWS<4 was considered as low acuity, and ED-MEWS≥4 as high acuity). A survival analysis compared times to 30-day inpatient death between CFS categories (1-4: very fit to vulnerable, 5: mildly frail, 6: moderately frail, and 7-8: severely or very severely frail). RESULTS: There were 12,282 non-elective patient episodes (8202 first episodes, of which complete data was available for 5505). In a Cox proportional hazards model controlling for age, gender, Charlson Comorbidity Index, history of dementia, current cognitive concern, and discharging specialty (medical versus surgical), ED-MEWS≥4 (HR=2.87, 95% CI: 2.27-3.62, p<0.001), and CFS 7-8 (compared to CFS 1-4, HR=2.10, 95% CI: 1.52-2.92, p<0.001) were independent predictors of survival time. CONCLUSIONS: We found frailty and AIS independently associated with inpatient mortality after adjustment for confounders. Hospitals may find it informative to undertake large scale assessment of frailty (vulnerability), as well as AIS (stressor), in older patients admitted to hospital as emergencies.