PurposeFrailty is a condition characterized by a reduced ability to adapt to external stressors because of a reduced physiologic reserve, which contributes to the high risk of postoperative mortality in hip fracture patients. This study aims to investigate how frailty is associated with the specific causes of mortality in hip fracture patients.MethodsAll adult patients in Sweden who suffered a traumatic hip fracture and underwent surgery between 2008 and 2017 were eligible for inclusion. The Orthopedic Hip Frailty Score (OFS) was used to classify patients as non-frail (OFS 0), pre-frail (OFS 1), and frail (OFS ≥ 2). The association between the degree of frailty and both all-cause and cause-specific mortality was determined using Poisson regression models with robust standard errors and presented using incidence rate ratios (IRRs) with corresponding 95% confidence intervals (CIs), adjusted for potential sources of confounding.ResultsAfter applying the inclusion and exclusion criteria, 127,305 patients remained for further analysis. 23.9% of patients were non-frail, 27.7% were pre-frail, and 48.3% were frail. Frail patients exhibited a 4 times as high risk of all-cause mortality 30 days [adj. IRR (95% CI): 3.80 (3.36–4.30),p< 0.001] and 90 days postoperatively [adj. IRR (95% CI): 3.88 (3.56–4.23),p< 0.001] as non-frail patients. Of the primary causes of 30-day mortality, frailty was associated with a tripling in the risk of cardiovascular [adj. IRR (95% CI): 3.24 (2.64–3.99),p< 0.001] and respiratory mortality [adj. IRR (95% CI): 2.60 (1.96–3.45),p< 0.001] as well as a five-fold increase in the risk of multiorgan failure [adj. IRR (95% CI): 4.99 (3.95–6.32),p< 0.001].ConclusionFrailty is associated with a significantly increased risk of all-cause and cause-specific mortality at 30 and 90 days postoperatively. Across both timepoints, cardiovascular and respiratory events along with multiorgan failure were the most prevalent causes of mortality.