Background: Frailty is among the most serious global public health challenges due to the rapid increase in the ageing population and age-associated declines in health. We aimed to validate hospital frailty risk score (HFRS) for its ability to predict prolonged hospital length of stay, 28-day unplanned readmission, repeated admission, and mortality in older people over a 15-year follow-up period. Methods: We linked data from the Australian Longitudinal Study on Women's Health (ALSWH) with hospital admission and National Death Index datasets to identify admitted patients and death dates. This study included patients with an index unplanned admission resulting in an overnight hospital stay in 2001���2016 and aged 75���95 years at the time of admission. HFRS and Charlson comorbidity index (CCI) were calculated from the hospital data using the International Statistical Classification of Diseases, Australia Modification (ICD-10-AM) diagnostic codes. Results: Of 2740 older women aged 75 years and over with unplanned admission, the proportions of patients with low, intermediate, and high frailty risks were 77.15 % (n = 2114), 20.95 % (n = 574), and 1.90 % (n = 52), respectively. The 15-year follow-up revealed that high frailty risk patients increased 5-fold in 2015 (15.67 % patients, mean age = 92.26 years) compared to 2001 (2.56 % patients, mean age = 77.96 years). Prolonged hospital length of stay was higher in the intermediate (AOR = 2.86, 95 %CI: 2.26, 3.62) and high frailty risk group (AOR = 4.26, 95 %CI: 2.32, 7.63) compared to the low frailty risk group. Frailty risk was not associated with unplanned or repeated hospital admission. However, the intermediate frailty risk group (AHR = 1.78, 95 %CI: 1.47, 2.17) and the high frailty risk group (AHR = 4.17, 95 %CI: 2.00, 8.66) had a significant risk of mortality compared to the low frailty risk group. Conclusions: This study confirms the ability of HFRS to identify older, frail people at higher risk of prolonged hospital length of stay and increased mortality risk. However, we did not observe a significant association between HFRS and 28-day unplanned readmission or repeated hospital admission.