Background: Frailty is a state of increased vulnerability to stressors, associated to poor health outcomes. The aim of this study was to design and introduce a Frailty Index (FI; according to the age-related accumulation of deficit model) in a large cohort of hospitalized older persons, in order to benefit from its capacity to comprehensively weight the risk profile of the individual.Methods: Patients aged 65 and older enrolled in the REPOSI register from 2010 to 2016 were considered in the present analyses. Variables recorded at the hospital admission (including socio-demographic, physical, cognitive, functional and clinical factors) were used to compute the FI. The prognostic impact of the FI on in-hospital and 12-month mortality was assessed.Results: Among the 4488 patients of the REPOSI register, 3847 were considered eligible for a 34-item FI computation. The median FI in the sample was 0.27 (interquartile range 0.21-0.37). The FI was significantly predictive of both in-hospital (OR 1.61, 95% CI 1.38-1.87) and overall (HR 1.46, 95% CI 1.32-1.62) mortality, also after adjustment for age and sex.Conclusions: The FI confirms its strong predictive value for negative outcomes. Its implementation in cohort studies (including those conducted in the hospital setting) may provide useful information for better weighting the complexity of the older person and accordingly design personalized interventions.